Sahih Muslim 2528

Kuraib reported that Umm Fadl, daughter of Harith, sent him (Fadl, i.e. her son) to Mu'awiya in Syria. I (Fadl) arrived in Syria, and did the needful for her. It was there in Syria that the month of Ramadan commenced. I saw the new moon (of Ramadan) on Friday. I then came back to Medina at the end of the month. Abdullah b. 'Abbas (Allah be pleased with him) asked me (about the new moon of Ramadan) and said:When did you see it? I said: We saw it on the night of Friday. He said: (Did) you see it yourself? I said: Yes, and the people also saw it and they fasted and Mu'awiya also fasted, whereupon he said: But we saw it on Saturday night. So we will continue to fast till we complete thirty (fasts) or we see it (the new moon of Shawwal). I said: Is the sighting of the moon by Mu'awiya not valid for you? He said: No; this is how the Messenger of Allah (ﷺ) has commanded us. Yahya b. Yahya was in doubt (whether the word used in the narration by Kuraib) was Naktafi or Taktafi