Musannaf Ibn Abi Shayba 3:4499

Hushaym narrated to us, he said: Abd al-Rahman ibn Ishaq informed us, from Abd al-Malik ibn Umayr, who said: Ibn Mas’ud used to perform Witr with three [rak'ahs]. He would recite in each rak'ah of them three surahs from the end of the Mufassal.