Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) says: the Prophet (peace and blessings be upon him) used to offer eight voluntary rak'ahs at night, and twelve rak'ahs during the day.
Abu Ya'la recorded it; its narrators are the narrators of the Sahih, except Asim bin Hamzah, and he is thiqah (trustworthy) and thabt (precise).