Seeing the New Moon - Section 2 Mishkat al-Masabih 1977
Translation
‘Ammar b. Yasir said, "He who fasts on a day about which he is doubtful has disobeyed Abul Qasim"* *i.e., the Prophet.Abu Dawud, Tirmidhi, Nasa’i, Ibn Majah and Darimi transmitted it.