The Prophet (p) received intelligence that the Banu Ghatafan tribe were mobilising troops in order to attack the Muslim community.
Kitab al-tabaqat al-kabir – Ibn Sa’d:
“THE SARIYYAH OF ABU QATADAH IBN RIB’I AL-ANSARI TOWARDS KHUDRAH THE TERRITORY OF MUHARIB
then (occurred) the sariyyah of Abu Qatadah Ibn Rib’I al-Ansari towards Khudrah the territory of Muharib in Najd, in Sha’ban of the eighth year from the hijrah of the Apostle of llah. They (narrators) said: The Apostle of Allah sent Abu Qatadah with fifteen men against the Ghatafan; and ordered him to make a surprise attack on the enemy. He marched in the night and remained concealment in the day. He made an onslaught on a large concentration of the enemy which he surrounded. …” 
In Sha‘ban month 8 A.H., news reached the Prophet [pbuh] of amassing troops by Bani Ghatfan, still outside the domain of Islam. He urgently summoned Abu Qatadah and sent him at the head of fifteen men to discipline those outlaws. It took fifteen days to teach them an unforgettable lesson. … [Rahmat-ul-lil’alameen 2/233; Talqeeh Fuhoom Ahl Al-Athar p.33]” 
May I remind readers, the Banu Ghatafan were a very hostile people who had attacked and killed Muslims before:
 Kitab al-tabaqat al-kabir, by Ibn Sa’d, volume 2, page 163 – 164
 Sealed Nectar (‘Ar-Raheeq Al-Makhtum’), By Safi-ur-Rahman Al-Mubarakpuri, page 247