Hadhrat Jaabir Radiallahu Anhu narrates that during the Battle of Hunayn when Rasulullaah Sallallahu Alayhi Wasallam saw the Sahabah Radiallahu Anhum dispersing (when caught by surprise), he instructed Hadhrat Abbaas Radiallahu Anhu to call for the Ansaar and those who pledged allegiance beneath the tree (at Hudaybiyyah). They all responded saying, "At your service! At your service!" In fact, even those who were unable to get their camels to turn (towards the call because the camels were too frightened), threw on their armours, grabbed their swords and shields and hurried (on foot) towards the call. Soon a hundred Sahabah Radiallahu Anhum had gathered around Rasulullaah Sallallahu Alayhi Wasallam.
The enemy attacked them and the fight started.
While the first call had been for all the Ansaar, a second was made for the Khazraj tribe in particular because they were unwavering in battle. Rasulullaah Sallallahu Alayhi Wasallam peered out to see his cavalry and when he saw that they were fighting in earnest, he said, "Now is the time to heat the pebbles."
Hadhrat Jaabir Sallallahu Alaihi Wasallam continues. He says, "By Allaah! The Sahabah Radiallahu Anhum had hardly returned to the battle when (the Kuffaar were already defeated and) prisoners were being marched to Rasulullaah Sallallahu Alayhi Wasallam.
Allaah killed those Kuffaar who were to die and those destined to be defeated were defeated. Allaah then gave all the wealth and children of the enemy to Rasulullaah Sallallahu Alayhi Wasallam as booty." - Bayhaqi. as quoted in AI Bidaayah wan Nihaayah
Another narration from Hadhrat Abbaas Sallallahu Alaihi Wasallam states that Rasulullaah Sallallahu Alayhi Wasallam instructed Hadhrat Abbaas Radiallahu Anhu to call for the men of the acacia tree (those who pledged allegiance beneath the tree at Hudaybiyyah). Hadhrat Abbaas Sallallahu Alaihi Wasallam says, By Allaahl When they heard my voice, they returned to Rasulullaah Sallallahu Alayhi Wasallam just as a cow returned to her calves (when she senses that they are in danger). They were all calling out, 'Yaa Labbaikaa! Yaa Labbaikaa! (At your serviceI At your service!) - Muslim, as quoted in AI Bidaayah wan Nihaayah