Musannaf Ibn Abi Shayba 3:833

Abu Bakr narrated to us, he said: Ali ibn Mushir narrated to us, from al-Shaybani, from al-Walid ibn al-Ayzar, from Abu Amr al-Shaybani, from Abdullah ibn Mas’ud, who said: I asked the Messenger of Allah, "Which deed is best?" He said, "Prayer at its time."