Sunan Ibn Majah, Chapter: The Book of Fasting

that he heard Muawiyah bin Abu Sufyan on the pulpit saying: "The Messenger of Allah (saw) used to say from the pulpit, before the month of Ramadan: 'Fasting will begin on such and such a day, but we are going to start fasting earlier, so whoever wants to start fasting earlier (i.e., in Shaban), let him do so, and whoever wants to wait until Ramadan begins, let him do so.'" Daif