Abu Dawud Book 2, Hadith Number 1346.
Chapter : On the number of rakahs of the prayer at night.
Narated By ‘Aisha : The Apostle of Allah (PBUH) used to observe the witr prayer with nine rak’ahs. Then he used to pray seven rak’ahs (of witr prayer). He would pray two rak’ahs sitting after the witr in which he would recite the Qur’an (sitting). When he wished to bow, he stood up and bowed and prostrated.
Abu Dawud said: These two traditions have been transmitted by Khalid b. ‘Abd Allah al-Wasiti. In his version, he said: ‘Alqamah b. Waqqas said: O mother, how did he pray two rak’ahs? He narrated the tradition to the same effect.