We came to ʿAli bin Abi Talib ؓ and said: O Ameer al-Muʿmineen, why donʿt you tell us about the voluntary prayer of the Messenger of Allah ﷺ? He said: Who among you can do that? They said: We will take what we can do. He said: The Messenger of Allah ﷺ used to pray sixteen rakʿahs during the day apart from the Prescribed prayers.
تُحَدِّثُنَا عَنْ صَلاةِ النَّبِيِّ ﷺ تَطَوُّعَهُ؟ فَقَالَ وَأَيُّكُمْ يُطِيقُهُ؟ قَالُوا نَأْخُذُ مِنْهُ مَا أَطَقْنَا قَالَ كَانَ رَسُولُ اللهِ ﷺ يُصَلِّي مِنَ النَّهَارِ سِتَّ عَشْرَةَ رَكْعَةً سِوَى الْمَكْتُوبَةِ