Musannaf Ibn Abi Shayba 28:1163

Waki’ narrated to us, he said: Ali bin Salih narrated to us, from Uthman bin al-Mughirah al-Thaqafi, from Malik bin Jarir al-Hadrami, from Ali bin Abi Talib, he said: Fleeing from the battlefield is among the major sins.