the Messenger of Allah said: "Do no precede the month of Ramadan by fasting a day or two before it, unless it is the case of a man who normally performs some fast, then let him fast it."
Jami` at-Tirmidhi Book 8, Hadith 4
← Previous Hadith the Prophet said: "Do not precede the month with a day nor with two days, unless that fast falls on a day that one of you would have (normally) fasted. Fast with its sighting and break fast with its sighting, and if it is cloudy, then count for thirty days, and then break (the fast)."

Jami` at-Tirmidhi Book 8, Hadith 3 Next Hadith → "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)."

Jami` at-Tirmidhi Book 8, Hadith 5