Musnad Ahmad 27:943

Hassan narrated to us, Ibn Lahi'ah narrated to us, Abu Tu'mah narrated to us, that he said: I was with Ibn Umar when a man came to him and said: "O Abu Abd al-Rahman, I am strong enough to fast while traveling." Ibn Umar said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "Whoever does not accept the concession of Allah, upon him is a sin like the mountains of Arafat."