From Sahih Muslim (Hadith 1763):
Ibn 'Umar reported Allah's Messenger (ﷺ) 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
Reference: Muslim 1763