That man and the people of his city should not fast as an atonement except that they know (for certain) that the people of a certain city of Muslims had fasted on Sunday. In that case they should keep fast as an atonement.
Sunan Abi Dawud Book 14, Hadith 21
← Previous Hadith That Umm al-Fadl, daughter of al-Harith, sent him to Mu'aqiyah in Syria. He said: I came to syria and performed her work. The moon of Ramadan appeared while I was in Syria. We sighted the moon on the night of Friday. When I came to Median towards the end of the month (of Ramadan), Ibn 'Abbas asked me about the moon. He said: When did you sight the moon ? I said: I sighted it on the night of Friday. He asked: Did you sight it yourself ? I said: Yes, and the people sighted it. They fasted and Mu'awiyah also fasted. He said: But we sighted it on the night of saturday. Since then we have been fasting until we complete thirty days or we sight it. Then I said: Are the sighting of the moon by Mu'awiyah and his fasts not sufficient for us? He replied: No. The Messenger of Allah (ﷺ) commanded us to do so.

Sunan Abi Dawud Book 14, Hadith 20 Next Hadith → AbuIshaq reported on the authority of Silah: We were with Ammar on the day when the appearance of the moon was doubtful. (The meat of) goat was brought to him. Some people kept aloof from (eating) it. Ammar said: He who keeps fast on this day disobeys AbulQasim (i.e. the Prophet) (ﷺ).

Sunan Abi Dawud Book 14, Hadith 22