That the Prophet (ﷺ) entered upon her on Friday while she was fasting. He asked: Did you fast yesterday ? She said: No. He again asked: Do you intend to fast tomorrow ? She said: No. He said: So break your fast.
Sunan Abi Dawud Book 14, Hadith 110
← Previous Hadith The Prophet (ﷺ) said: Do not fast on Saturday except what has been made obligatory on you; and if one of you can get nothing but a grape skin or a piece of wood from a tree, he should chew it.

Sunan Abi Dawud Book 14, Hadith 109 Next Hadith → When it was mentioned to Ibn Shihab (al-Zuhri) that fasting on Saturday had been prohibited, he would say: This is a Himsi tradition.

Sunan Abi Dawud Book 14, Hadith 111