Yahya related to me from Malik, from his uncle Abu Suhayl from his father that Umar ibn al-Khattab wrote to Abu Musa saying that he should pray dhuhr when the sun had started to decline, asr when the sun was still pure white before any yellowness had entered it maghrib when the sun had set, and to delay isha as long as he did not sleep, and to pray subh when the stars were all visible and like a haze in the sky and to read in it two long suras from the mufassal.
Muwatta Malik Book 1, Hadith 7
← Previous Hadith Yahya related to me from Malik from Nafi', the mawla of Abdullah ibn Umar, that Umar ibn al-Khattab wrote to his governors saying, "The most important of your affairs in my view is the prayer. Whoever protects it and observes it carefully is protecting his deen, while whoever is negligent about it will be even more negligent about other things." Then he added, "Pray dhuhr any time from when the afternoon shade is the length of your forearm until the length of your shadow matches your height. Pray asr when the sun is still pure white, so that a rider can travel two or three farsakhs before the sun sets. Pray maghrib when the sun has set. Pray isha any time from when the redness in the western sky has disappeared until a third of the night has passed - and a person who sleeps, may he have no rest, a person who sleeps, may he have no rest. And pray subh when all the stars are visible and like a haze in the sky."

Muwatta Malik Book 1, Hadith 6 Next Hadith → Yahya related to me from Malik from Hisham ibn Urwa from his father that Umar ibn al-Khattab wrote to Abu Musa al-Ashari that he should pray asr when the sun was still pure white so that a man could ride three farsakhs (before maghrib) and that he should pray Isha during the first third of the night, or, if he delayed it, then up until the middle of the night, and he warned him not to be forgetful.

Muwatta Malik Book 1, Hadith 8