Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1631
Zaid b. Arqam reported that the Messenger of Allah (may peace be upon him) went out to the people of Quba' and saw them observing prayer; upon this he said: The prayer of the penitent should be observed when the young weaned camels feel heat of the sun.