Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1633
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.