'Ammar bin Yasir (May Allah be pleased with them) reported:
He who observes the fast on a doubtful day, has in fact disobeyed Abul-Qasim, (i.e., Messenger of Allah (ﷺ)).[Abu Dawud and At- Tirmidhi]. (DE)
App reference (DE): Book 9, Hadith 237 (DE)
Arabic/English book reference (DE): Book 9, Hadith 1227 (DE)