مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3261
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn 'Abbas (رضي الله عنه) said: The Messenger of Allah (ﷺ) was treated with cupping and he gave him (the cupper) his fee. If it were haram, he would not have given it to him.
Musnad Ahmad 3284its isnad is saheeh, al-Bukhari (2279)]