Sahih Bukhari Volume 001, Book 008, Hadith Number 451.
Narated By Abu Huraira : The Prophet sent some horsemen to Najd and they brought a man called Thumama bin Uthal from Bani Hanifa. They fastened him to one of the pillars of the mosque. The Prophet came and ordered them to release him. He went to a (garden of) date-palms near the mosque, took a bath and entered the, mosque again and said, "None has the right to be worshipped but Allah an Muhammad is His Apostle (i.e. he embraced Islam)."