Sayyiduna Zubayb bin Tha'labah al-Anbari (may Allah be pleased with him) says: I heard the Messenger of Allah (peace and blessings be upon him) saying: "Whoever is obliged to free a slave from the descendants of Isma'il, let him free one from Banu al-Anbar."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5279