Al-`Abbas bin `Abdul-Muttalib (RA) said: “I said: ‘O Messenger of Allah (ﷺ), teach me something that I may ask Allah, [Mighty and Sublime] for.’ He (ﷺ) said: ‘Ask Allah for Al-`Āfiyah.’ Then I remained for a day, then I came and said: ‘O Messenger of Allah (ﷺ), teach me something that I may ask Allah for.’ So he (ﷺ) said to me: “O Abbas, O uncle of the Messenger of Allah! Ask Allah for Al-`Āfiyah in the world and in the Hereafter.” [1] Narrated by Abu Bakr Siddiq (RA): The Prophet (ﷺ) said (sitting) on the pulpit, "Seek forgiveness and healing from Allah, for no one has been given anything better than healing after Yaqeen." [2]

Reference: [1] Sahih (Albani). Tirmidhi: 3514 [2] Bukhari, Al-Adabul Mufrad, 724