Surah Al-Falaq and Surah A-Naas (two last ‘Qul’ surahs). These two surahs are also referred to as “Mu’wadaitain”,
Aisha ( رظي الله عنها) narrated that whenever Allah’s Apostle (ﷺ) became sick, he would recite Surah Al-Falaq and Surah An-Naas and then blow his breath over his body. When he became seriously ill, I used to recite (these two Surahs) and rub his hands over his body for its blessings.
[Narrarted by Saheeh Al-Bukhari: The Book of the Virtues of the Qur’aan, 5016]