مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2723
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated from Ibn 'Abbas (رضي الله عنه) that a man said: O Messenger of Allah, is Hajj every year? He said: “Rather it is one Hajj required of each person. If I said yes, it would be (obligatory) every year.”
Musnad Ahmad 2741A saheeh hadeeth; this is a da'eef isnad]