Riyad us-Salihin Hadith 1248

Reported Mujibah Al-Bahiliyah on the authority of her father or uncle that he visited the Messenger of Allah (ﷺ) once and then went to see him again after a year. His appearance had totally changed. He asked the Messenger of Allah (ﷺ) if he had recognized him. The Messenger of Allah (ﷺ) asked, "Who are you?" He replied: "I am Al- Bahili who visited you last year." The Messenger of Allah (ﷺ) said, "You were quite handsome, what has changed your appearance so much?" He replied, "Since my departure from here, I have not eaten anything except at night." The Messenger of Allah (ﷺ) remarked, "You have put yourself to torture. Observe Saum (fasting) during the Month of Patience (i.e., Ramadan) and fast one day from each month." He submitted, "Permit me to observe more voluntary fasts because I have capacity to do so." The Messenger of Allah (ﷺ) said, "Then observe fasts for two days in every month." He said, "Permit me to observe more." The Messenger of Allah (ﷺ) said, "Observe fasts on three days in every month." He requested that he should be allowed to observe more fasts. The Messenger of Allah (ﷺ) said, "Fast three days during the sacred months and omit fasting for three days alternately." He (ﷺ) joined his three fingers together and left them apart while repeating this sentence thrice. [Abu Dawud].