I asked the Prophet ﷺ about fasting.He said, "Fast one day each month."I said, "Messenger of Allah, I am capable of more."The Messenger of Allah ﷺ said, "I am capable of more, I am capable of more. Fast two days each month."I said, "Messenger of Allah, give me more."The Messenger of Allah ﷺ said, "Give me more, give me more. Three days each month."
حدثنا وكيع حدثنا الأسود بن شيبان
سألت النبي ﷺ عن الصوم فقال صم من الشهر يوما قال قلت يا رسول الله إني أقوى فقال رسول الله ﷺ إني أقوى إني أقوى صم يومين من كل شهر قال قلت يا رسول الله زدني فقال رسول الله ﷺ زدني زدني ثلاثة أيام من كل شهر
Its isnad is sahih. Abu Nawfal was deemed reliable by Ibn Hibban and Ibn Main, and the hadith has already appeared in similar form through the narration of Abdullah bin Amr and others. See Nasai 4/212, in the Book of Fasting, chapter on additional fasting, and Muslim 2/817, no. 1159. It has also already appeared at 14976 and 15043 with a similar account.