Sahih Muslim Book 4, Hadith Number 1630.

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, 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.