Musnad Ahmad 25:466

'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: He—meaning 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: 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—or: you would not have been able to act upon it. Hajj is once; whoever does more, it is a voluntary act."