A report reached the Prophet (p) that the Banu Qudah tribe were intending to attack Madinah. Hence, Prophet Muhammed (p) sent out Amr Ibn al-As to see if this was true. Reaching the area where the enemy was encamped, Amr Ibn al-As seen the preparations made by them. They were a lot more than the Muslims.
Amr Ibn al-As sent news to the Prophet (p) that they needed extra fighters. With reinforcements coming, the Muslims fought against them, the enemy all fled.
Kitab al-tabaqat al-kabir, by Ibn Sa’d:
“THE SARIYYAH OF AMR IBN AL-AS TOWARDS DHAT AL-SALASIL, WHICH IS BEYOND WADI AL-QURA.
Then (occurred) the sariyyah of Amr Ibn Al-As towards Dhat al-Salasil, which is beyond Wadi al-Qura. (The distance) between is and al-Madinah being ten days (journey). It took place in Jumada al-Akhira of the eighth year from the Hijrah of the Apostle of Allah.
They (narrators) said: IT (REPORT) REACHED THE APOSTLE OF ALLAH THAT A PARTY OF THE QUDA’AH HAD ASSEMBLED, AND INTENDED TO ADVANCE TO THE SEAT OF THE APOSTLE. Thereupon the Apostle of Allah, summoned Amr Ibn al-As, and prepared for him a white flag and a black banner. He sent him wit three hundred selected Muhajirss and al-Ansars. With them there were thirty horses.
He ordered him to ask for aid for those of the Baliyy, the Udhrah and Balqayn by whom he would pass. He marched in the night and REMAINED IN CONCEALMENT IN THE DAY. When he was close to the people, the news reached him that WERE HEAVY ODSS, so he sent Rafi Ibn Makith al-Juhami to the Apostle of Allah, asking him to send help.
He (Prophet) sent Abu Ubaydah Ibn al-Jarrah with two hundred men. He prepared a flag for him and sent the notables of the Muhajirs and al-Ansar, among whom were Abu Bakr and Umar.
He odderred him to join Amr and that they should be in agreement and should not disagree. … He resumed his march and reached the lands of the Baliyy, gathering information about the various routes. Then he reached the borders of their territory and the land of the Udhrah and the Balqayn, HE MET THE ENEMIES WHO HAD ASSEMBLED.
The Muslims attacked them. They fled to their land and dispersed. Then he returned and sent Awf Ibn Malik al-Ashj’ai as a messenger to the Apostle of Allah. He (Awf) informed him of their safe return and what had happened in the fighting. …” 
Maulana Wahiduddin Khan:
“Dhat al-Salasil was a place in the Syrian Desert, occupied by the Ghassanid and Kalb tribes to which the Prophet sent an expedition under the leadership of Amr ibn Al-As. When the latter reached there, and saw the preparations being made by the enemy, he realized that his own force was too weak to do battle with them.
He thereupon set up a camp, and sent a message to the Prophet asking for reinforcements. The Prophet then prepared an additional force of 200 Muhajirs, which was despatched under the leadership of Abu UBaydah ibn al-Jarrah.” 
 Kitab al-tabaqat al-kabir, by Ibn Sa’d, volume 2, page 162 – 163
 Muhammad: A Prophet for All Humanity By Maulana Wahiduddin Khan, page 399