Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1630
Zaid b. Arqam, on seeing some people praying in the forenoon, said: They well know that prayer at another time than this is more excellent, for Allah's Messenger (may peace be upon him) said: The prayer of those who are penitent is observed when your weaned camels feel the heat of the sun.