Narrated Ibn `Abbas: Once the Prophet got his blood out (medically) and paid that person who had done it. If it had been illegal, the Prophet would not have paid him.
Hadith In Arabic Text :
حدثنا مسدد، حدثنا خالد هو ابن عبد الله حدثنا خالد، عن عكرمة، عن ابن عباس رضى الله عنهما قال احتجم النبي صلى الله عليه وسلم وأعطى الذي حجمه، ولو كان حراما لم يعطه.
Chapter : Sales and Trade – كتاب البيوع