"We were with 'Ammar and a roast sheep was brought and he said: 'Eat.' One of the people turned away and said: 'I am fasting 'Ammar said: Whoever fasts on the day concerning which there is doubt, has disobeyed Abu Al-Qasim."' '
Sunan an-Nasa'i Book 22, Hadith 99
← Previous Hadith "The Messenger of Allah used to fast Sha`ban and Ramadan, and he made sure to fast on Mondays and Thursday."

Sunan an-Nasa'i Book 22, Hadith 98 Next Hadith → "I entered upon 'Ikrimah on the day concerning which there was doubt as to whether it was Ramadan or Shaban, and he was eating bread, vegetables and milk. He said: 'Come and eat.' I said: 'I am fasting.' He adjured me by Allah to break my fast. I said Subhan-Allah twice. When I saw that he was insisting, I went forward and said: 'Give me what you have.' He said: 'I heard Ibn 'Abbas say: The Messenger of Allah said: 'Fast when you see it (the crescent) and stop fasting when you see it, and if clouds or darkness prevent you from seeing it, then complete the number of days of Shaban, and do not fast ahead of the month, and do not join Ramadan to a day of Shaban." '

Sunan an-Nasa'i Book 22, Hadith 100