Sayyidah Aishah (may Allah be pleased with her) said: Quraysh and those who followed their religion used to stand at Muzdalifah, and they were called the Hums, while all the rest of the Arabs used to stand at Arafah. When Islam came, Allah Almighty commanded His Prophet (peace and blessings be upon him) to go to Arafat, stand there, and depart from there. That is the saying of Allah Almighty: "Then depart from where the people depart." [Surah al-Baqarah: 199]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9450