Musnad Ahmad 27:41

Isma'il narrated to us, Ayyub informed us, from Nafi', from Ibn 'Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The month is only twenty-nine [days]. So do not fast until you see it, and do not break your fast until you see it. But if it is obscured from you, then estimate it." Nafi' said: When twenty-nine days of Sha'ban had passed, 'Abdullah would send someone to look for it. If it was seen, then that was it. If it was not seen, and neither clouds nor dust obstructed the view of it, he would wake up having broken his fast. But if clouds or dust obstructed the view of it, he would wake up fasting.