It is narrated from Sayyiduna Ali (may Allah be pleased with him) that throughout the day, the voluntary prayers (nawafil) of the Prophet (peace and blessings be upon him) were sixteen rak‘ahs.
Hadith Referenceمسند احمد / مسند الخلفاء الراشدين / 885
Hadith Gradingحکم دارالسلام:صحيح، شريك - وإن كان سيء الحفظ - قد توبع