It is narrated from Sayyiduna Zayd bin Arqam (may Allah be pleased with him); he said: I heard the Messenger of Allah (peace and blessings be upon him) say: "O Allah, forgive the Ansar, and the offspring of the Ansar, and the offspring of their offspring."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5084