Mujahid reported this from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both), and to the best of my knowledge he narrated it from the Prophet (peace and blessings be upon him). The Prophet (peace and blessings be upon him) said: "Hajj is (standing at) Arafat."
Al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains Khusayf, whom Yahya bin Ma'in and others declared thiqah (trustworthy), while Ahmad and others declared him da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الحج / 5543
Hadith Gradingمحدثین:إسنادہ ضعیف
Hadith Takhrij«أخرجه المعجم الأوسط للطبراني برقم: 5840»