Abu Dawud Book 2, Hadith Number 1357.
Chapter : On the number of rakahs of the prayer at night.
Narated By ‘Abd Allah b. Abi Qais : I asked ‘Aisha: How many rak’ahs would the Apostle of Allah (PBUH) pray observing the witr? She said: He used to observe the witr with four and three, six and three, eight and three and ten and three rak’ahs never observing less than seven or more than thirteen.
The narrator Ahmad added in his version: He would not observe the witr with two rak’ahs before the dawn. I asked: With what would he observe the witr? She replied: He would never leave it. The version of Ahmad does not mention the words “six and three (rak’ahs).”