Sala bin Zufar al-Absi صلة بن زفر · The Book on Fasting

"We were with Ammar bin Yasir when a roasted sheep was brought and he said: 'Eat.' Someone among the people said: 'I am fasting.' So Ammar said: 'Whoever fasts on a day in which there is doubt, then he has disobeyed Abul-Qasim (pbuh)."