It was narrated from Abu Nawfal bin Abi 'Aqrab that his father said:

"I asked the Messenger of Allah about fasting and he said: 'Fast one day of the month.' I said: 'Fast one day of the month.' I said: 'O Messenger of Allah, let me do more, let me do more.' He said: 'Let me do more, let mo do more; I am able for it.' Then the Messenger of Allah fell silent until I thought that he was going to refuse my request. Then he said: 'fast three days of each month."'

أَخْبَرَنَا عَمْرُو بْنُ عَلِيٍّ، قَالَ حَدَّثَنِي سَيْفُ بْنُ عُبَيْدِ اللَّهِ، مِنْ خِيَارِ الْخَلْقِ قَالَ حَدَّثَنَا الأَسْوَدُ بْنُ شَيْبَانَ، عَنْ أَبِي نَوْفَلِ بْنِ أَبِي عَقْرَبٍ، عَنْ أَبِيهِ، قَالَ سَأَلْتُ رَسُولَ اللَّهِ ﷺ عَنِ الصَّوْمِ فَقَالَ " صُمْ يَوْمًا مِنَ الشَّهْرِ " . قُلْتُ يَا رَسُولَ اللَّهِ زِدْنِي زِدْنِي . قَالَ " تَقُولُ يَا رَسُولَ اللَّهِ زِدْنِي زِدْنِي يَوْمَيْنِ مِنْ كُلِّ شَهْرٍ " . قُلْتُ يَا رَسُولَ اللَّهِ زِدْنِي زِدْنِي إِنِّي أَجِدُنِي قَوِيًّا . فَقَالَ " زِدْنِي زِدْنِي أَجِدُنِي قَوِيًّا " . فَسَكَتَ رَسُولُ اللَّهِ ﷺ حَتَّى ظَنَنْتُ أَنَّهُ لَيَرُدُّنِي قَالَ " صُمْ ثَلاَثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ " .