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.  

أَنَّ عُمَرَ بْنَ الْخَطَّابِ كَتَبَ إِلَى أَبِي مُوسَى الأَشْعَرِيِّ أَنْ صَلِّ الْعَصْرَ وَالشَّمْسُ بَيْضَاءُ نَقِيَّةٌ قَدْرَ مَا يَسِيرُ الرَّاكِبُ ثَلاَثَةَ فَرَاسِخَ وَأَنْ صَلِّ الْعِشَاءَ مَا بَيْنَكَ وَبَيْنَ ثُلُثِ اللَّيْلِ فَإِنْ أَخَّرْتَ فَإِلَى شَطْرِ اللَّيْلِ وَلاَ تَكُنْ مِنَ الْغَافِلِينَ