It was narrated from Kuraib that

Ummul-Fadl bint al-Harith sent him to Mu'awiyah in Syria. He said: So I came to Syria and did her errand, then the month of Ramadan began when I was in Syria, and we saw the new moon on the night before Friday. Then I came to Madinah at the end of the month and 'Abdullah bin 'Abbas (رضي الله عنه) asked me about the new moon. He said: when did you see the new moon? I said: We saw it on the night before Friday. He said: Did you see it? I said: Yes and the people saw it and fasted, and Mu'awiyah fasted. He said: But we saw it on the night before Saturday, we will carry on fasting until we complete thirty days or we see it (the new moon of Shawwal). I said: Is not the sighting and fasting of Mu'awiyah sufficient for you? He said: No, this is the command of the Prophet (ﷺ).