Prior to this decision, the Ansaar intended to execute Hazrat Abbaas (radhiyallahu ‘anhu) among the other prisoners. The news of their intention reached Nabi (sallallahu ‘alaihi wasallam) and it affected him to the point where he expressed, “I was unable to sleep last night due to worrying about my uncle, ‘Abbaas.” When Hazrat ‘Umar (radhiyallahu ‘anhu) heard this, he asked Rasulullah (sallallahu ‘alaihi wasallam), “Should I go to the Ansaar and speak to them to hand ‘Abbaas over to me?”
Rasulullah (sallallahu ‘alaihi wasallam) granted permission to Hazrat ‘Umar (radhiyallahu ‘anhu), and he thus approached the Ansaar (radhiyallahu ‘anhum) and requested them to release Hazrat ‘Abbaas (radhiyallahu ‘anhu). Initially, they did not know that the request was on behalf of Rasulullah (sallallahu ‘alaihi wasallam) and so refused to release him. When they refused, Hazrat ‘Umar (radhiyallahu ‘anhu) asked them, “If releasing him will be a means of you acquiring the pleasure of Rasulullah (sallallahu ‘alaihi wasallam), will you then do so?” They immediately responded, “If it will please Rasulullah (sallallahu ‘alaihi wasallam), then we will wholeheartedly release him and you may take him immediately!”
Hazrat ‘Umar (radhiyallahu ‘anhu) then told them that he was sent by Rasulullah (sallallahu ‘alaihi wasallam), and that Rasulullah (sallallahu ‘alaihi wasallam) was unable to sleep the night on account of seeing his uncle in difficulty and distress. Hazrat ‘Umar (radhiyallahu ‘anhu) then took Hazrat ‘Abbaas (radhiyallahu ‘anhu) from the Ansaar, and as he proceeded with him, he pleaded with him saying, “O ‘Abbaas! Please accept Islam! If you accept Islam, your accepting Islam will be more beloved to me than even my own father, Khattaab, accepting Islam. The reason for your accepting Islam pleasing me more than my own father, Khattaab, accepting Islam, is that this will greatly please Rasulullah (sallallahu ‘alaihi wasallam).” Shortly thereafter, Hazrat ‘Abbaas (radhiyallahu ‘anhu) accepted Islam.
(Al-Bidaayah-wan-Nihaayah 3/319)