Hadhrat Aasim bin Umar bin Qataadah Radiallahu Anhu narrates that when the Ansaar gathered to pledge their allegiance to Rasulullah Sallallahu Alayhi Wasallam Hadhrat Abbaas bin Ubaadah bin Nadhla Radiallahu Anhu who belonged to the Banu Saalim bin Awf tribe said, "0 assembly of the Khazraj! Do you know on what conditions you are pledging allegiance to this man?" When they replied in the affirmative, he said, 'You are pledging to wage war against every fair and dark skinned person! if you fear that you will surrender him to his enemies when your wealth starts getting destroyed and your leaders start getting killed, you better leave him now. By Allaah! If you do that (desert him afterwards), it would mean disgrace in this world as well as in the Aakhirah. However, if you feel that you would be able to fulfil the claim you have made to him despite the destruction of your wealth and the killing of your leaders, you should take him with you. By Allaah! This would mean the best for you in this world as well as in the Aakhirah."
The Ansaar confirmed, "We are prepared to take him even though it means the destruction of our wealth and the deaths of our leaders. 0 Rasulullah Sallallahu Alayhi Wasallam What will we gain if we fulfil our pledge?" Rasulullah Sallallahu Alayhi Wasallam replied by saying, "Jannah." They then asked Rasulullah Sallallahu Alayhi Wasallam to stretch out his hand and when he did, they pledged their allegiance to him. (1)
According to a narration reported by Hadhrat Abdullaah bin Ka'b Radiallahu Anhu, Rasulullah Sallallahu Alayhi Wasallam said to the Ansaar (after they had pledged their allegiance), "Leave for your camps." It was then that Hadhrat Abbaas bin Ubaadah Radiallahu Anhu said, "0 Rasulullah Sallallahu Alayhi Wasallam I swear by the Being Who has sent you with the truth that if you wish, we shall attack the people at Mina with our swords by tomorrow." Rasulullah Sallallahu Alayhi Wasallam said, 'You have not been commanded to do this. You may however leave for your camps." (2)
(1) Ibn Is'haaq as reported in A/ Bidaaya wan Nihaays((Vol.3 Pg. 162).
(2) Ibn Is'haaq as reported in AIBidaaya wan Nihaaya (Vol.3 Pg. 164)
Source: Hayaatus Sahaabah