Musannaf Ibn Abi Shayba 36:135

Waki’ narrated to us, from Isra’il, from Abu Ishaq, from Harithah ibn Mudhrib al-‘Abdi, from ‘Ali, who said: The distinguishing mark of the companions of the Messenger of Allah (may Allah bless him and grant him peace) on the day of Badr was white wool.