Musnad Ahmad 25:1661

Rawh narrated to us, Zam'ah narrated to us, from Ibn Shihab, from Abu Sinan al-Du'ali, from Ibn Abbas, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Allah the Almighty has prescribed Hajj for you." Al-Aqra' ibn Habis said: "Every year, O Messenger of Allah?" He said: "Rather, it is one Hajj. And had I said 'yes,' it would have become obligatory."