"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.
Sunan an-Nasa'i Book 22, Hadith 345
← Previous Hadith "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."'

Sunan an-Nasa'i Book 22, Hadith 344 Next Hadith → "The Messenger of Allah said to Mu'adh when he sent him to Yemen: 'You are going to some of the People of the book. When you come to them, call them to testify that there is none worthy of worship except Allah and that Muhammad is the Messenger of Allah. If they obey you in that, then tell them that Allah, the Mighty and Sublime, has enjoined on them a charity (Zakah) to be taken from their rich and given to their poor. If they obey you in that, then beware of the supplication of the oppressed person."'

Sunan an-Nasa'i Book 23, Hadith 1