Yahya related to me from Malik from Yazid ibn Ziyad that Abdullahibn Rafi, the mawla of Umm Salama, the wife of the Prophet, may Allahbless him and grant him peace, asked Abu Hurayra about the time of theprayer. Abu Hurayra said, "Let me tell you. Pray dhuhr when the lengthof your shadow matches your height, asr when your shadow is twice yourheight, maghrib when the sun has set, isha in the first third of thenight, and subh in the very first light of dawn," i.e. when the dawnhas definitely come.
Arabic/English book reference: Book 1, Hadith 9