Mishkat al-Masabih 585
It is told of ‘Umar b. al-Khattab that he wrote to his governors, “The most important matter which concerns you in my opinion is prayer; whoever observes it and is attentive to it will guard his religion, but whoever neglects it will be more neglectful of other things." Thereafter he wrote telling them to observe the moon prayer in the period when the shade was a cubit long up to the time when a man’s shadow was as long as himself; the afternoon prayer when the sun was high, white and clear, when there was still time for a rider to go two or three leagues before sunset:
the sunset prayer after the sun had set; the night prayer between the ending of the twilight and the passing of a third of the night (adding three times “if one lies down to sleep may his eye not sleep”); and the morning prayer when the stars were still visible and out in abundance. 

Malik transmitted it.
وَعَنْ عُمَرَ بْنِ الْخَطَّابِ رَضِيَ اللَّهُ عَنْهُ: أَنَّهُ كَتَبَ إِلَى عُمَّالِهِ إِنَّ أَهَمَّ أُمُورِكُمْ عِنْدِي الصَّلَاة فَمن حَفِظَهَا وَحَافَظَ عَلَيْهَا حَفِظَ دِينَهُ وَمَنْ ضَيَّعَهَا فَهُوَ لِمَا سِوَاهَا أَضْيَعُ ثُمَّ كَتَبَ أَنْ صلوا الظّهْر إِذا كَانَ الْفَيْءُ ذِرَاعًا إِلَى أَنْ يَكُونَ ظِلُّ أَحَدِكُمْ مِثْلَهُ وَالْعَصْرَ وَالشَّمْسُ مُرْتَفِعَةٌ بَيْضَاءُ نَقِيَّةٌ قَدْرَ مَا يَسِيرُ الرَّاكِبُ فَرْسَخَيْنِ أَوْ ثَلَاثَةً قبل مغيب الشَّمْس وَالْمغْرب إِذا غربت الشَّمْسُ وَالْعِشَاءَ إِذَا غَابَ الشَّفَقُ إِلَى ثُلُثِ اللَّيْلِ فَمَنْ نَامَ فَلَا نَامَتْ عَيْنُهُ فَمَنْ نَامَ فَلَا نَامَتْ عَيْنُهُ فَمَنْ نَامَ فَلَا نَامَتْ عَيْنُهُ وَالصُّبْحَ وَالنُّجُومُ بَادِيَةٌ مُشْتَبِكَةٌ. رَوَاهُ مَالك