Mukhtasar Bukhari

Narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that his father (Sayyiduna Umar, may Allah be pleased with him) sent him to bring his horse which was with an Ansari man. (On the way) he saw that the Messenger of Allah (peace and blessings of Allah be upon him) was taking the pledge of allegiance from the people under a tree, and Sayyiduna Umar (may Allah be pleased with him) was not aware of this. So he (i.e., Abdullah bin Umar, may Allah be pleased with them both) pledged allegiance to the Messenger of Allah (peace and blessings of Allah be upon him), then went to get the horse and brought it to Sayyiduna Umar (may Allah be pleased with him), who was (wearing armor), putting on his weapons. Then Sayyiduna Abdullah (may Allah be pleased with him) told him that the people were pledging allegiance to the Messenger of Allah (peace and blessings of Allah be upon him) under the tree. (Then) both of them went until Sayyiduna Umar (may Allah be pleased with him) pledged allegiance to the Messenger of Allah (peace and blessings of Allah be upon him). This is (the) incident (because of) which people say that Abdullah bin Umar (may Allah be pleased with them both) accepted Islam before Sayyiduna Umar (may Allah be pleased with him).