Sunan An-Nasa'i

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."" (Sahih) The end of what the Shaikh had about fasting, all praise is due to Allah the Lord of the worlds