Imam Malik's Muwatta, Chapter: The Times of Prayer

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.