Sahih Muslim Book 4, Hadith Number 1633.

Chapter : Forenoon prayer should be offered when it is sufficiently hot.

Salim reported on the authority of his father that a person asked the Apostle of Allah (may peace be upon him) about the night prayer. He said: It consists of pairs of rak’ahs, but if one fears morning is near, he should make it an odd number by praying one rak’ah.