Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him) narrates: We came back from Hudaybiyah while we were with the Messenger of Allah (peace and blessings be upon him), and we pledged allegiance to him (peace and blessings be upon him) at the foot of a tree, and I pledged allegiance first. The Prophet (peace and blessings be upon him) said to me: "O Salamah, pledge allegiance to me." I said: O Prophet of Allah, by Allah, I have already pledged allegiance among the first. He (peace and blessings be upon him) said: "Even so." So I pledged allegiance again. Then the Messenger of Allah (peace and blessings be upon him) saw me while I had no shield with me, and the narrator mentioned the rest of the hadith.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6214