Musnad Ahmad 25:783

'Affan narrated to us, Sulayman ibn Kathir Abu Dawud al-Wasiti narrated to us, he said: I heard Ibn Shihab narrating from Abu Sinan, from Ibn 'Abbas, he said: The Messenger of Allah (peace and blessings of Allah be upon him) delivered a sermon to us and said: "O people, Hajj has been prescribed for you." He said: Then al-Aqra' ibn Habis stood up and said: Is it every year, O Messenger of Allah? He said: "If I had said yes, it would have become obligatory, and if it had become obligatory, you would not have acted upon it, and you would not have been able to act upon it. Hajj is once; whoever does more, it is voluntary."