There came to Messenger of Allah ﷺ 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."  

أَتَى رَسُولَ اللَّهِ ﷺ نَفَرٌ مِنْ عُرَيْنَةَ فَأَسْلَمُوا وَبَايَعُوهُ وَقَدْ وَقَعَ بِالْمَدِينَةِ الْمُومُ وَهُوَ الْبِرْسَامُ ثُمَّ ذَكَرَ نَحْوَ حَدِيثِهِمْ وَزَادَ وَعِنْدَهُ شَبَابٌ مِنَ الأَنْصَارِ قَرِيبٌ مِنْ عِشْرِينَ فَأَرْسَلَهُمْ إِلَيْهِمْ وَبَعَثَ مَعَهُمْ قَائِفًا يَقْتَصُّ أَثَرَهُمْ