Yahya related to me from Malik from Yazid ibn Ziyad that Abdullah ibn Rafi, the mawla of Umm Salama, the wife of the Prophet ﷺ asked Abu Hurayra about the time of the prayer. Abu Hurayra said, "Let me tell you. Pray dhuhr when the length of your shadow matches your height, asr when your shadow is twice your height, maghrib when the sun has set, isha in the first third of the night, and subh in the very first light of dawn," i.e. when the dawn has definitely come.  

أَنَّهُ سَأَلَ أَبَا هُرَيْرَةَ عَنْ وَقْتِ الصَّلاَةِ فَقَالَ أَبُو هُرَيْرَةَ أَنَا أُخْبِرُكَ صَلِّ الظُّهْرَ إِذَا كَانَ ظِلُّكَ مِثْلَكَ وَالْعَصْرَ إِذَا كَانَ ظِلُّكَ مِثْلَيْكَ وَالْمَغْرِبَ إِذَا غَرَبَتِ الشَّمْسُ وَالْعِشَاءَ مَا بَيْنَكَ وَبَيْنَ ثُلُثِ اللَّيْلِ وَصَلِّ الصُّبْحَ بِغَبَشٍ يَعْنِي الْغَلَسَ