Yahya related to me from Malik from Nafi', the mawla of Abdullah ibn Umar, that Umar ibn al-Khattab wrote to his governorssaying, "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 aboutother things." Then he added, "Pray dhuhr any time from when theafternoon shade is the length of your forearm until the length of yourshadow 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 theredness in the western sky has disappeared until a third of the nighthas passed - and a person who sleeps, may he have no rest, a personwho sleeps, may he have no rest. And pray subh when all the stars arevisible and like a haze in the sky."
Arabic/English book reference: Book 1, Hadith 6