Asim bin Damrah reports that once a person asked Sayyiduna Ali (may Allah be pleased with him) how the Prophet (peace and blessings be upon him) used to perform voluntary prayers (nawafil). He replied that during the whole night, the Prophet (peace and blessings be upon him) would perform sixteen rak‘ahs of voluntary prayers.
Hadith Referenceمسند احمد / مسند الخلفاء الراشدين / 1234
Hadith Gradingحکم دارالسلام:إسناده قوي ، غير أن قوله فيه : «من الليل» خطأ، والصحيح «من النهار» انظر رقم : 650 و 1642