Yahya related to me from Malik, from his uncle Abu Suhayl fromhis father that Umar ibn al-Khattab wrote to Abu Musa saying that heshould pray dhuhr when the sun had started to decline, asr when thesun was still pure white before any yellowness had entered it maghribwhen 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 inthe sky and to read in it two long suras from the mufassal.
Arabic/English book reference: Book 1, Hadith 7