It is narrated from Sayyiduna Samurah bin Jundub (may Allah be pleased with him); he said: The Messenger of Allah (peace and blessings be upon him) appointed the battle cry of the Muhajirun as 'O Banu Abd al-Rahman', that of the Khazraj as 'O Banu Abdullah', and that of the Aws as 'O Banu Ubaydullah'; and when we were alarmed, he named our horses 'the horses of Allah'.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 7071