Muwatta Malik 9 — The Times of Prayer

Yazid ibn Ziyad Narrated from:

Abdullah ibn Rafi, the mawla of Umm Salama, the wife of the Prophet, may Allah bless him and grant him peace, 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.