Musannaf Ibn Abi Shayba 4:86

Waki’ narrated to us, from Muslim ibn Yazid, from his father, who said: Ali ibn Abi Talib used to say to Ibn al-Tayyah (al-Nabbah): "Has the sun set?" He would say: "Do not be hasty." Then he would say: "Has the sun set?" When he said: "Yes," he would break his fast, then descend and pray.