It is narrated from Sayyiduna Abu Mahdhurah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) assigned to Banu Abd al-Muttalib the service of giving water to the pilgrims (siqayah), and assigned to Banu Abd al-Dar the keeping of the keys of the Ka'bah (hijabah), and assigned the adhan to us and to our mawali (freed clients).
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6707