Sahih Muslim Book 16, Hadith Number 4135.
Chapter : Pertaining to the combatants and apostates.
Anas reported: There came to Allah’s Messenger (may peace be upon him) some people from ‘Uraina. They embraced Islam and swore allegiance to him and there had spread at that time pleurisy. The rest of the hadith is the same (but with this addition): “There were by his (the Prophet’s) side about twenty young men of the Ansar; he sent them (behind) them (culprits), and he also sent along with them one expert in following the track so that he might trace their footprints.”