Musannaf Ibn Abi Shayba 3:4433

Hushaym narrated to us, he said: Al-Hajjaj informed us, from 'Ata', that Mu'awiyah performed Witr with one Rak'ah, and that was disapproved of by some. So Ibn 'Abbas was asked about it, and he said: "He has hit upon the Sunnah."