Night Prayer.

Bukhari :: Book 2 :: Volume 16 :: Hadith 106

Narrated Ibn 'Abbas:

Once I passed the night in the house of Maimuna (his aunt). I slept across the bed while Allah's Apostle and his wife slept length-wise. The Prophet slept till midnight or nearly so and woke up rubbing his face and recited ten verses from Surat "Aal-Imran." Allah's Apostle went towards a leather skin and performed ablution in the most perfect way and then stood for the prayer. I did the same and stood beside him. The Prophet put his right hand on my head, twisted my ear and then prayed two Rakat five times and then ended his prayer with Witr. He laid down till the Muadh-dhin came then he stood up and offered two Rakat (Sunna of Fajr prayer) and then went out and offered the Fajr prayer.

(See Hadith No. 183, Vol. 1)

Bukhari :: Book 2 :: Volume 16 :: Hadith 107

Narrated 'Abdullah bin 'Umar :

The Prophet said, "Night prayer is offered as two Rakat followed by two Rakat and so on, and if you want to finish it, pray only one Raka which will be Witr for all the previous Rakat." Al-Qasim said, "Since we attained the age of puberty we have seen some people offering a three-Rakat prayer as Witr and all that is permissible. I hope there will be no harm in it."