Maliks Muwatta Book 1, Hadith Number 6.

Section : The Times of Prayer.

Yahya related to me from Malik from Nafi from 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.”