Abu Dawud Book 2, Hadith Number 1351.

Chapter : On the number of rakahs of the prayer at night.

Narated By Ibn ‘Abbas : I spent a night with my maternal aunt Maimunah. The Apostle of Allah (PBUH) came after the evening had come. He asked: Did the boy pray? She said: Yes. Then he lay down till a part of night had passed as much as Allah willed; he got up, performed ablution and prayed seven or five rak’ahs, observing witr with the. He uttered the salutation only in the last of them.