Yahya related to me from Malik from Hisham ibn Urwa from hisfather that Umar ibn al-Khattab wrote to Abu Musa al-Ashari that heshould pray asr when the sun was still pure white so that a man couldride three farsakhs (before maghrib) and that he should pray Ishaduring the first third of the night, or, if he delayed it, then upuntil the middle of the night, and he warned him not to be forgetful.
Arabic/English book reference: Book 1, Hadith 8