Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1647
Ibn 'Umar reported Allah's Messenger (may peace be upon him) as saying: The night prayer consists of pairs and when you see the approach of dawn, make this number odd by one rak'ah. It was said to Ibn 'Umar: What does the (word) pair imply? He said: (It means) that salutation is uttered after every two rak'ahs.