It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) prayed wrapped in a single garment. Someone asked Abu Zubair whether this referred to obligatory prayer? He replied: This includes both obligatory and non-obligatory prayers.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 14344