Musnad Ahmad 32:149

It was read to Sufyan: I heard ‘Asim, from Anas, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, established a pact of alliance between the Emigrants (Muhajirun) and the Helpers (Ansar) in our house." Sufyan said: "It is as if he means: He established brotherhood (mu’akhah) between them."