1.     Faj’r Salah is when the true dawn breaks i.e. when whiteness marks the horizon until sunrise.

'Abdullah b. 'Amr reported (RA): The Messenger of Allah (ﷺ) said: The time of the noon prayer is when the sun passes the meridian and a man's shadow is the same (length) as his height, (and it lasts) as long as the time for the afternoon prayer has not come; the time for the afternoon prayer is as long as the sun has not become pale; the time of the evening prayer is as long as the twilight has not ended; the time of the night prayer is up to the middle of the average night and the time of the morning prayer is from the appearance of dawn, as long as the sun has not risen; but when the sun rises, refrain from prayer for it rises between the horns of the devil. [Muslim]

2.     Faj’r Salah does not start with false dawn i.e. first the light appears length-wise (like a vertical pillar) in the horizon (false dawn) followed by darkness and then light appears bredth-wise which is the true dawn.

Samura b. Jundub (RA)reported Allah's Messenger (ﷺ) as saying: The Adhan of Bilal should not mislead you nor the whiteness (of the pillar) of dawn, for it is not the whiteness of the true dawn, but that of the false dawn which is vertical like a pillar and you can eat food till the streaks of whiteness spread like it. [Muslim]

3.     It is Mustahab to pray Faj’r Salah in Asfaar i.e. when the light becomes clear.

Raf’e Ibn Khadeej (RA)reported that Allah's Messenger (ﷺ) said: Make the Faj’r Salah at the shining of the dawn as your reward will be greater. [Tirmidhi]

Ibrahim Nakhai (RA) said, the comapanions of the Prophet (ﷺ) have not agreed on anything as they agreed on praying Faj’r when it is clear and shining. [Ibn Abi Shaybah]

This has also been narrated from Ali Ibn Abi Talib (RA), Abdullah Ibn Masood (RA), Hussain Ibn Ali (RA), Abu Darda (RA), Umar Ibn Abdul-Aziz (RA), Zaid Ibn Aslam (RA), Muhammad Ibn Sireen (RA), Alqamah (RA), Suwayd Ibn Ghafalah (RA),  Said Ibn Jubair (RA) and others. [Ibn Abi Shaybah]

4.     Zohar Salah starts from when the sun has gone past the zenith until the time for Asar has started.

'Abdullah b. 'Amr reported (RA): The Messenger of Allah (ﷺ) said: The time of the noon prayer is when the sun passes the meridian and a man's shadow is the same (length) as his height, (and it lasts) as long as the time for the afternoon prayer has not come; the time for the afternoon prayer is as long as the sun has not become pale; the time of the evening prayer is as long as the twilight has not ended; the time of the night prayer is up to the middle of the average night and the time of the morning prayer is from the appearance of dawn, as long as the sun has not risen; but when the sun rises, refrain from prayer for it rises between the horns of the devil. [Muslim]

5.     During very hot weather, Zohar Salah should be slightly delayed (until it is relatively cooler) and not prayed immediately or as soon as the time has started.

Narrated Abu Huraira (RA): The Prophet (ﷺ) said, "In very hot weather delay the Zuhr prayer till it becomes (a bit) cooler because the severity of heat is from the raging of Hell-fire. The Hell-fire of Hell complained to its Lord saying: O Lord! My parts are eating (destroying) one another. So Allah allowed it to take two breaths, one in the winter and the other in the summer. The breath in the summer is at the time when you feel the severest heat and the breath in the winter is at the time when you feel the severest cold."[Bukhari]

6.     Asar Salah starts when the time for Zohar Salah has expired i.e. when the shadow of an object is equal to twice its length (at midday) according to Imam Abu Haneefa (RA) and when the shadow is equal to its length (at midday) according to Imam Abu Yusuf (RA) and Imam Muhammad (RA).

Abdullah ibn Rafi (RA), the mawla of Umm Salama, the wife of the Prophet (ﷺ) asked Abu Hurayra (RA) about the time of the prayer. Abu Hurayra (RA) said, "Let me tell you. Pray dhuhr when the length of your shadow matches your height, asr when your shadow is twice your height, maghrib when the sun has set, isha in the first third of the night, and subh in the very first light of dawn," i.e. when the dawn has definitely come. [Muwatta Imam Malik]

Narrated Abu Al-Mahh (RA): We were with Buraida in a battle on a cloudy day and he said, "Offer the 'Asr prayer early as the Prophet (ﷺ) said, "Whoever leaves the 'Asr prayer, all his (good) deeds will be annulled."[Bukhari]

7.     Asar Salah shouldn’t be delayed to such an extent that the times gets very close to sunset.

Al-Ala ibn Abd ar-Rahman (RA) said, "We visited Anas ibn Malik (RA) after dhuhr and he (RA) stood up and prayed asr. When he had finished his prayer, we mentioned doing prayers early in their time, or he mentioned it, and he said that he had heard the Messenger of Allah(ﷺ) say, the prayer of the hypocrites, the prayer of the hypocrites, the prayer of the hypocrites is that one of them sits until the sun becomes yellow and is between the horns of Shaytan, or on the horn of Shaytan, and then gets up and rattles off four rakas, hardly remembering Allah in them at all.' "[Muwatta Imam Malik]

8.     Maghrib Salah starts as soon as the sunsets until the dissapearnce of the twilight.

'Abdullah b. 'Amr reported (RA): The Messenger of Allah (ﷺ) said: The time of the noon prayer is when the sun passes the meridian and a man's shadow is the same (length) as his height, (and it lasts) as long as the time for the afternoon prayer has not come; the time for the afternoon prayer is as long as the sun has not become pale; the time of the evening prayer is as long as the twilight has not ended; the time of the night prayer is up to the middle of the average night and the time of the morning prayer is from the appearance of dawn, as long as the sun has not risen; but when the sun rises, refrain from prayer for it rises between the horns of the devil. [Muslim]

Note:There is a difference of opinion on this matter with Imam Abu Yusuf (RA) and Imam Muhammad (RA) considering twilight to be the redness on the horizon while Imam Abu Haneefa (RA) considering it to be whitness on the horizon which comes after the redness has disappeared.

9.     Maghrib Salah should be prayed as soon as the time starts.

Salamah Ibn Akwah (RA) said, “The Messenger of Allah (ﷺ) used to do the Maghrib Salah immediately after the sun had set when its upper side would disappear. [Abu Dawud]

10.                        Esha Salah starts after Maghrib Salah has ended i.e. when the twilight has ended until the appearance of true dawn (i.e. beginning of Faj’r Salah).

'Abdullah b. 'Amr reported (RA): The Messenger of Allah (ﷺ) said: The time of the noon prayer is when the sun passes the meridian and a man's shadow is the same (length) as his height, (and it lasts) as long as the time for the afternoon prayer has not come; the time for the afternoon prayer is as long as the sun has not become pale; the time of the evening prayer is as long as the twilight has not ended; the time of the night prayer is up to the middle of the average night and the time of the morning prayer is from the appearance of dawn, as long as the sun has not risen; but when the sun rises, refrain from prayer for it rises between the horns of the devil. [Muslim]

Aisha (RA) reported , “One night the Messenger of Allah (ﷺ) prayed the Isha Salah after most of the night had gone and most of the people in the Masjid had fallen asleep. Then he came out, prayed and said, “This would be the proper time if it were not a hardship on my nation”. [Bukhari]

11.                        However, the preferred time for Esha Salah is upto one third of the night.

Abu Hurairah (RA) reported that the Messenger of Allah (ﷺ) said, “If it were not to be a hardship on the Ummah, I would have ordered them to delay the Esha Salah until a third of the night had passed. [Tirmidhi]

12.                        Sleeping before Isha and talking afterwards is disapproved and disliked.

Narrated Abu-l-Minhal (RA): My father and I went to Abi Barza Al-Aslami (RA) and my father said to him, "Tell us how Allah's Apostle (ﷺ) used to offer the compulsory congregational prayers." He said, "He (ﷺ) used to pray the Zuhr prayer, which you call the first prayer, as the sun declined at noon, the 'Asr at a time when one of us could go to his family at the farthest place in Medina while the sun was still hot. (The narrator forgot what Abu Barza had said about the Maghrib prayer), and the Prophet preferred to pray the 'Isha' late and disliked to sleep before it or talk after it. And he used to return after finishing the morning prayer at such a time when it was possible for one to recognize the person sitting by his side and he (the Prophet) used to recite 60 to 100 'Ayat' (verses) of the Qur'an in it." [Bukhari]

13.                        It is prohibited to perform Salah when the sun is rising, at miday and when the sun is setting.

Uqba b. 'Amir said (RA): There were the times at which Allah's Messenger (ﷺ)  forbade us to pray, or bury our dead: When the sun begins to rise till it is fully up, when the sun is at its height at midday till it passes over the meridian, and when the sun draws near to setting till it sets. [Muslim]

14.                        It is prohibited to perform Nafl Salah after Faj’r Salah until the sun has risen and after the Asar Salah until the sun has set.

Narrated Qaza'a Maula (RA), (freed slave of) Ziyad: I heard Abu Said Al-khudri narrating four things from the Prophet (ﷺ)  and I appreciated them very much. He said, conveying the words of the Prophet:

a)      "A woman should not go on a two day journey except with her husband or a Dhi-Mahram.

b)    No fasting is permissible on two days: 'Id-ul-Fitr and 'Id-ul-Adha.

c)     No prayer after two prayers, i.e. after the Fajr prayer till the sunrises and after the 'Asr prayer till the sun sets.

d)    Do not prepare yourself for a journey except to three Mosques, i.e. Al-Masjid-AI-Haram, the Mosque of Aqsa (Jerusalem) and my Mosque." [Bukhari]