Musannaf Ibn Abi Shayba 3:5908

Abu Mu'awiyah narrated to us, from Hajjaj, from Abu Ishaq, from al-Harith, from Ali, who said: I prayed the prayer of fear with the Prophet, may Allah bless him and grant him peace, two rak'ahs [and] two rak'ahs, except for the Maghrib, for he prayed it as three.