Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1632
Ibn 'Umar reported that a person asked the Messenger of Allah (may peace be upon him) about the night prayer. The Messenger of Allah (may peace be upon him) said: Prayer during the night should consist of pairs of rak'ahs, but if one of you fears morning is near, he should pray one rak'ah which will make his prayer an odd number for him.