Hazrat Tha‘labah (ra) bin Abi Malik relates that on every occasion, Hazrat Sad (ra) bin Ubadah would hold the flag on behalf of the Holy Prophet (sa). However, whenever the battle would commence, Hazrat Ali (ra) bin Abi Talib would take hold of it.
(Ali Ibn al-Athir, Usd al-Ghabah fi Ma‘rifat al-Sahabah, Vol. 4, Dhikr Ali bin Abi Talib [Beirut, Lebanon: Dar al-Fikr, 2003], 93)