Sahih Muslim Book 4, Hadith Number 1644.

Ibn ‘Umar reported: A person called (the attention) of the Messenger of Allah (may peace be upon him) as he was in the mosque, and said: Messenger of Allah, how should I make the rak’ahs of the night prayer an odd number? Upon this the Messenger of Allah (way peace he upon him) said: He who prays (night prayer) he should observe it in pairs, but if he apprehends the rise of morning, he should observe one rak’ah; that would make the number odd (for the rak’ahs) observed by him. This was narrated by Abd Kuraib ‘Ubaidullah b. ‘Abdullah and Ibn ‘Umar did not make mention of it.