I was present with the Messenger of Allah ﷺ when some people came and asked him about the Hajj. The Messenger of Allah ﷺ said, "Hajj is Arafah. Whoever reaches Arafah on the night before the dawn of the night of Jam has completed his Hajj."
أخبرنا إسحق بن إبراهيم قال أنبأنا وكيع قال حدثنا سفيان عن بكير بن عطاء
شهدت رسول الله ﷺ فأتاه ناس فسألوه عن الحج فقال رسول الله ﷺ الحج عرفة فمن أدرك ليلة عرفة قبل طلوع الفجر من ليلة جمع فقد تم حجه
[Sahih] Related by Ibn Majah, Chapters on the Rites of Hajj, chapter of one who comes to Arafah before Fajr on the night of gathering, no. 3015, from the hadith of Waki, through this chain. Ibn Khuzayma graded it sahih, no. 2822, as did Hakim, 1/278, 463, 464, and Dhahabi concurred with him. Sufyan Thawri stated explicitly that he heard it directly, as will come, no. 3047. It was also related by Abu Dawud, no. 1949, and Tirmidhi, nos. 889, 890, from the hadith of Sufyan Thawri, through this chain.