سنن النسائي

It was narrated from Abu Nawfal bin Abi 'Aqrab, from his father, that he asked the Prophet (ﷺ) 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 (ﷺ) said: "I am able, I am able." He did not want to increase it, but when I insisted, the Messenger of Allah (ﷺ) said: "Fast three days of each month." (Sahih) The end of what the Shaikh had about fasting, all praise is due to Allah the Lord of the worlds.