Abu Dawud Book 2, Hadith Number 1321.
Chapter : The prayer at the night is to be offered in pairs of rakahs each time.
Narated By ‘Abd Allah b. ‘Umar : A man asked the Apostle of Allah (PBUH) about the prayer at night. The Apostle of Allah (PBUH) said: Prayer during the night should consist of pairs of rak’ahs, but if one of you fears the morning is near he should pray one rak’ah which will make his prayer an odd number for him.