Al-Qasim Abu Abd al-Rahman narrates that he heard Sayyiduna Mu'awiyah (may Allah be pleased with him) say: The Messenger of Allah (peace and blessings be upon him) used to stand on the pulpit one day before Ramadan and say: "The fast begins on such-and-such a day, and we are starting early, so whoever wishes to start early may do so, and whoever wishes to leave it may leave it."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16929