The Book Pertaining to the Oath, for Establishing the Responsibility of Murders, Fighting, Requital and Blood-Wit (Kitab Al-Kitab Al-Qasama wa'l-Muharaba wa'l-Qisas wa'l-Diyat)
Muslim :: Book 16 : Hadith 4135
Anas reported: There came to Allah's Messenger (may peace be upon him) some ponple 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."