From Sahih Bukhari (Hadith 462):
Narrated Abu Huraira:The Prophet (ﷺ) 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 (ﷺ) 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 and Muhammad is His Apostle" (i.e. he embraced Islam)
Reference: Bukhari 462