Musannaf Ibn Abi Shayba 3:2158

Marwan ibn Mu'awiyah narrated to us, from Mansur, from Hayyan, who said: Ja'far ibn Kathir ibn al-Muttalib al-Sahmi narrated to me, saying: Abu al-Darda' said: "O people, beware of turning (your heads) during prayer, for there is no prayer for the one who turns. And if you are overcome regarding the voluntary (prayers), then do not be overcome regarding the obligatory (prayers)."