Abu Hurairah (RAA) narrated that The Messenger of Allah (ﷺ) forbade fasting on the day of Arafah for the one who is actually at Arafah (i.e. performing Hajj). Related by the five Imams except for At-Tirmidhi. Ibn Khuzaimah and al-Hakim graded it as Sahih. (IT)
English reference (IT): Book 5, Hadith 714 (IT)
Arabic reference (IT): Book 5, Hadith 694 (IT)