“The Messenger of Allah (ﷺ) forbade anticipating the fast by fasting one day before sighting (of the crescent).”
Sunan Ibn Majah Book 7, Hadith 9
← Previous Hadith “We were with ‘Ammar on the day concerning which there was some doubt. A (roasted) sheep was brought and some of the people moved away. ‘Ammar said: ‘Whoever is fasting on this day has disobeyed Abu Qasim (ﷺ).’”

Sunan Ibn Majah Book 7, Hadith 8 Next Hadith → “The Messenger of Allah (ﷺ) 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 Sha’ban), let him do so, and whoever wants to wait until Ramadan begins, let him do so.’”

Sunan Ibn Majah Book 7, Hadith 10