Musnad Ahmad 25:1651

Rawh narrated to us, Muhammad ibn Abi Hafsah narrated to us, Ibn Shihab narrated to us, from Abu Sinan, from Ibn Abbas: That al-Aqra’ ibn Habis asked the Messenger of Allah (may Allah bless him and grant him peace): "Is the Hajj every year?" He said: "No, rather it is one Hajj. Whoever performs Hajj after that, it is a voluntary act. If I had said 'yes,' it would have become obligatory, and if it had become obligatory, you would not have heard and you would not have obeyed."