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.  

أَنَّ عُمَرَ بْنَ الْخَطَّابِ كَتَبَ إِلَى أَبِي مُوسَى أَنْ صَلِّ الظُّهْرَ إِذَا زَاغَتِ الشَّمْسُ وَالْعَصْرَ وَالشَّمْسُ بَيْضَاءُ نَقِيَّةٌ قَبْلَ أَنْ يَدْخُلَهَا صُفْرَةٌ وَالْمَغْرِبَ إِذَا غَرَبَتِ الشَّمْسُ وَأَخِّرِ الْعِشَاءَ مَا لَمْ تَنَمْ وَصَلِّ الصُّبْحَ وَالنُّجُومُ بَادِيَةٌ مُشْتَبِكَةٌ وَاقْرَأْ فِيهَا بِسُورَتَيْنِ طَوِيلَتَيْنِ مِنَ الْمُفَصَّلِ  

[Machine] Pray Dhuhr when the sun starts to fade, and Asr when the sun turns pure white before it becomes yellow. Pray Maghrib after the sun sets, and Isha until you sleep. Pray Fajr when the stars are still visible, and recite in it two long chapters from the Quran.  

أَنْ صَلِّ الظُّهْرَ إِذَا زَالَتِ الشَّمْسُ وَالْعَصْرَ وَالشَّمْسُ بَيْضَاءُ نَقِيَّةٌ قَبْلَ أَنْ تَدْخُلْهَا صُفْرَةٌ وَالْمَغْرِبَ إِذَا غَرَبَتِ الشَّمْسُ وَالْعِشَاءُ مَا لَمْ تَنَمْ وَصَلِّ الصُّبْحَ وَالنُّجُومُ بَادِيَةٌ وَاقْرَأْ فِيهِ سُورَتَيْنِ طَوِيلَتَيْنِ مِنَ الْمُفَصَّلِ