Sahih Muslim Book 4, Hadith Number 1631.

Chapter : Prayer during the night and the number of the Rak’ahs which the Apostle (may peace be upon him) observed in the night and observance of one Rak’ah of Witr.

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.