Musnad Ahmad

Kuraib (may Allah have mercy on him) says that once Sayyida Umm al-Fadl bint al-Harith (may Allah be pleased with her) sent him to Sham (Syria). When I reached there, I completed my task, and while I was still in Sham, the new moon of Ramadan was sighted. We saw the moon on the night of Friday. At the end of the month, when I returned to Madinah, Sayyiduna Abdullah ibn Abbas (may Allah be pleased with them both) asked me about the task, and then the topic of the moon came up. He asked, "When did you people see the moon?" I replied, "On the night of Friday." He asked, "Did you see it yourself?" I said, "Yes! And other people also saw it. The people fasted upon seeing the moon, and Sayyiduna Amir Muawiyah (may Allah be pleased with him) also fasted accordingly." Sayyiduna Ibn Abbas (may Allah be pleased with them both) said, "But we saw the moon on the night of Saturday (the next day was Saturday), so we will continue to fast until we complete thirty days or see the moon." I said, "Can you not suffice with the sighting and fasting of Sayyiduna Amir Muawiyah (may Allah be pleased with him)?" He said, "No, this is what the Prophet (peace and blessings be upon him) commanded." (From here, the jurists have derived the issue of difference in moon sighting.)