the Messenger of Allah said: "Count the (the appearances of) the crescent of Sha'ban for Ramadan."
Jami` at-Tirmidhi Book 8, Hadith 6
← Previous 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 Next Hadith → the Messenger of Allah said: "Do not fast before Ramadan. Fast with its sighting, and break fast with its sighting, and if it is obscured from you, then complete thirty days."

Jami` at-Tirmidhi Book 8, Hadith 7