In the sixth year after Hijrah, during the month of Sha’baan, Rasulullah (sallallahu ‘alaihi wasallam) addressed Hazrat Abdur Rahmaan bin Auf (radhiyallahu ‘anhu) and said to him, “Make preparations to travel, as I am about to send you for an expedition, either today or tomorrow, Insha Allah.”
The following morning, when Hazrat Abdur Rahmaan bin Auf (radhiyallahu ‘anhu) came before Rasulullah (sallallahu ‘alaihi wasallam), he was wearing a black turban. Rasulullah (sallallahu ‘alaihi wasallam) untied his turban and retied it around his head, with the tail hanging behind his back. The length of the tail was approximately the length of four fingers. Rasulullah (sallallahu ‘alaihi wasallam) then said to him: “O Ibnu Auf, this is how you should wear the turban, as this is the way of the Arabs and it is better.”
Rasulullah (sallallahu ‘alaihi wasallam) appointed Hazrat Abdur Rahmaan bin Auf (radhiyallahu ‘anhu) as the leader of the army and hence told Hazrat Bilaal (radhiyallahu ‘anhu) to hand over the flag to him. Rasulullah (sallallahu ‘alaihi wasallam) then praised Allah Ta’ala, sent salutations upon himself, and said:
“O Ibnu Auf, take the flag, and set out in the path of Allah with the army. Fight against those people who disbelieve in Allah Ta’ala. Do not steal from the spoils of war, do not go against your pledges (which you make with the enemy), do not mutilate the bodies of the enemy, and do not kill children. This is the command of Allah Ta’ala and the way of His Nabi (sallallahu ‘alaihi wasallam) which has come to you.”
Thereafter, Rasulullah (sallallahu ‘alaihi wasallam) dispatched him in the path of Allah Ta’ala with an army of seven hundred men towards the tribe of Kalb, situated in a place called Dumat al-Jandal. Rasulullah (sallallahu ‘alaihi wasallam) then gave him the following advice saying:
“If the people of that place accept (Islam), then you should marry the daughter of their leader.”
Hazrat Abdur Rahmaan (radhiyallahu ‘anhu) thus departed with his army until he reached the people of Dumat al-Jandal. He remained there for three days, inviting them towards Islam.
On the third day, the leader of Dumat al-Jandal, Asbagh, who was a Christian, embraced Islam. Many other people from his tribe also accepted Islam. Those who chose not to accept Islam agreed to pay the jizyah.
Hazrat Abdur Rahmaan (radhiyallahu ‘anhu) then wrote a letter to Rasulullah (sallallahu ‘alaihi wasallam) informing him of the situation and reconfirming whether he should marry the daughter of their leader. Rasulullah (sallallahu ‘alaihi wasallam) replied to his letter saying that he should go ahead and marry the daughter of the leader, Asbagh. Hazrat Abdur Rahmaan (radhiyallahu ‘anhu) thus got married to his daughter whose name was Tumaadhir (radhiyallahu ‘anha).
(Bazzaar #6175, Majma’uz Zawaaid #9615, Sharhuz Zurqaani 3/133)