Musannaf Ibn Abi Shayba 3:951

Hafs narrated to us, from Abu al-Anbas ‘Amr ibn Marwan, who said: I asked my father, saying: "I have prayed with ‘Ali, so inform me how he used to pray [Maghrib]." He said: "He used to pray Maghrib when the disk [of the sun] had set."