Sahih Muslim Book 4, Hadith Number 1635.

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

‘Abdullah b. ‘Umar reported: A person asked the Apostle of Allah (may peace be upon him) as I stood between him (the Holy Prophet) and the inquirer and he said: Messenger of Allah, how is the night prayer? He (the Holy Prophet) said: It consists of pairs of rak’ahs, but if you apprehend morning, you should pray one rak’ah and make the end of your prayer as Witr. Then a person asked him (the Holy Prophet) at the end of the year and I was at that place near the Messenger of Allah (may peace be upon him); but I do not know whether he was the same person or another person, but he (the Holy Prophet) gave him the same reply.