From Sahih Muslim (Hadith 5737):
Uthman b. Abu al-'As Al-Thaqafi reported that he made a complaint of pain to Allah's Messenger (ﷺ) that he felt in his body at the time he had become Muslim. Thereupon Allah's Messenger (ﷺ) said:Place your hand at the place where you feel pain in your body and say Bismillah (in the name of Allah) three times and seven times A'udhu billahi wa qudratihi min sharri ma ajidu wa uhadhiru (I seek refuge with Allah and with His Power from the evil that I find and that I fear)
Reference: Muslim 5737