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.

وَعَنْ أَبِي هُرَيْرَةَ ‏- رضى الله عنه ‏- أَنَّ اَلنَّبِيَّ ‏- صلى الله عليه وسلم ‏-{ نَهَى عَنْ صَوْمِ يَوْمِ عَرَفَةَ بِعَرَفَةَ } رَوَاهُ اَلْخَمْسَةُ غَيْرَ اَلتِّرْمِذِيِّ, وَصَحَّحَهُ اِبْنُ خُزَيْمَةَ, وَالْحَاكِمُ, وَاسْتَنْكَرَهُ الْعُقَيْلِيُّ [1]‏ .‏