Mustadrak al-Hakim — Hadith 5757

It is narrated from Asim bin Umar, concerning the names of those Companions among the Muhajirun and the Ansar between whom the Messenger of Allah (peace and blessings be upon him) established brotherhood (mu'akhah), that he said: The Messenger of Allah (peace and blessings be upon him) established brotherhood between Sayyiduna Ammar bin Yasir and Sayyiduna Hudhayfah bin al-Yaman (may Allah be pleased with them both). Abdullah bin Ja'far said: Although Sayyiduna Hudhayfah (may Allah be pleased with him) was not able to take part in Badr, his Islam was very early. And they all agree that Sayyiduna Ammar bin Yasir (may Allah be pleased with him) took part in Badr, Uhud, the Trench (Khandaq) and all the battles alongside the Messenger of Allah (peace and blessings be upon him).