In Sunni references, there is a hadith saying that, “anyone who fasts Dhu l-Hijja 18, God regards the rewards of 6 months fasting for him” and that “this day is Eid Ghadir Khumm.”
Khaṭīb al-Baghdādī, Tārīkh al-Baghdād, vol. 8, p. 284.