Abu Da'ud أبو داود السجستاني الأزدي · The Book on Fasting

"A person from the offspring of Umm Hani narrated to me - I met one of the most virtuous among them, and his name was Ja'dah, and Umm Hani was his grandmother - he narrated to me from his grandmother that the Messenger of Allah entered upon her and asked for some drink, and he drank. Then he offered it to her and she drank it. Then she said: "O Messenger of Allah! I was fasting." So the Messenger of…

the Messenger of Allah said: "O Abu Dharr! When you fast three days out of a month, then fast the thirteenth, fourteenth, and fifteenth."

"I heard Mu'adhah saying to Aishah: 'Did the Messenger of Allah fast three days of every month?' She replied in the affirmative. So she said: 'Which of them would he fast?' She said: 'Which of them he would fast was not noticeable.'"

"I heard a freed slave of ours called Laila narrated from Umm Amarah bint Ka'b Al-Ansar, that the Prophet entered upon her and some food was brought to him. He said: 'Eat.' She said: 'I am fasting.' So the Messenger of Allah said: 'Indeed the angels send Salat upon the fasting person when (others) eat in his presence, until they finish.' And perhaps he said: 'Until they have eaten their fill.'"