Musnad Ahmad 2296

It was narrated that Ibn ‘Abbas (رضي الله عنه) said: He - meaning the Messenger of Allah (ﷺ) - addressed us and said: "O people, Hajj has been prescribed for you.” Al-Aqra` bin Habis stood up and said: Is it every year, O Messenger of Allah? He said: "If I said that, it would become obligatory, and if it became obligatory you would not do it - or you would not be able to do it. Hajj is once [in a lifetime], and whoever does more, it is voluntary.”