It was narrated from Abu Nawfal bin Abi 'Aqrab, from his father, that he asked the Prophet about fasting and he said:

"Fast one day of each month." He asked him for more, saying: "May my father and mother be ransomed for you, I am able." He said: "Fast two days of each month." He said" May my father and mother be sacrificed for you, O Messenger of Allah, I am able." The Messenger of Allah said: "I am able, I am able." He did not want to increase it, but when I insisted, the Messenger of Allah said: "Fast three days of each month."
The end of what the Shaikh had about fasting, all praise is due to Allah the Lord of the worlds.

أَخْبَرَنَا عَبْدُ الرَّحْمَنِ بْنُ مُحَمَّدِ بْنِ سَلاَّمٍ، قَالَ حَدَّثَنَا يَزِيدُ بْنُ هَارُونَ، قَالَ أَنْبَأَنَا الأَسْوَدُ بْنُ شَيْبَانَ، عَنْ أَبِي نَوْفَلِ بْنِ أَبِي عَقْرَبٍ، عَنْ أَبِيهِ، أَنَّهُ سَأَلَ النَّبِيَّ ﷺ عَنِ الصَّوْمِ فَقَالَ " صُمْ يَوْمًا مِنْ كُلِّ شَهْرٍ " . وَاسْتَزَادَهُ قَالَ بِأَبِي أَنْتَ وَأُمِّي أَجِدُنِي قَوِيًّا فَزَادَهُ قَالَ " صُمْ يَوْمَيْنِ مِنْ كُلِّ شَهْرٍ " . فَقَالَ بِأَبِي أَنْتَ وَأُمِّي يَا رَسُولَ اللَّهِ إِنِّي أَجِدُنِي قَوِيًّا . فَقَالَ رَسُولُ اللَّهِ ﷺ " إِنِّي أَجِدُنِي قَوِيًّا إِنِّي أَجِدُنِي قَوِيًّا " . فَمَا كَادَ أَنْ يَزِيدَهُ فَلَمَّا أَلَحَّ عَلَيْهِ قَالَ رَسُولُ اللَّهِ ﷺ " صُمْ ثَلاَثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ " .