The first day I witnessed in battle was the Day of the Trench.
أول يوم شهدته يوم الخندق.
Bukhari narrated it in the Book of Military Expeditions (4107), from Abdah bin Abdullah, who said, Abdul-Samad narrated to us, from Abdul-Rahman, who is Ibn Abdullah bin Dinar, from his father, that Ibn Umar mentioned it.Ibn Saad said, "The one carrying the banner of the migrants was Zayd bin Haritha, and the banner of the Ansar was carried by Saad bin Ubada." Tabaqat (3/67)