Maliks Muwatta Book 1, Hadith Number 8.

Section : The Times of Prayer.

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.