Musnad Ahmad 25:930

Sulayman ibn Dawud al-Hashimi narrated to us, Isma'il—meaning Ibn Ja'far—narrated to us, he said: Muhammad—meaning Ibn Abi Harmala—informed me, from Kurayb: That Umm al-Fadl bint al-Harith sent him to Mu'awiyah in al-Sham. He said: So I arrived in al-Sham and fulfilled her need. Then Ramadan began while I was in al-Sham, and we saw the crescent moon on the night of Friday. Then I arrived in Medina at the end of the month. Abdullah ibn Abbas asked me, then he mentioned the crescent moon and said: "When did you see the crescent moon?" I said: "We saw it on the night of 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 of Saturday, so we will continue to fast until we complete thirty [days] or we see it." I said: "Will you not suffice with the sighting of Mu'awiyah and his fasting?" He said: "No, this is how the Prophet, peace and blessings of Allah be upon him, commanded us."