When visiting the sick, one should recite the following dua:
لَا بَأْسَ طَهُوْرٌ إِنْ شَاءَ اللهُ
There is no need to worry, insha-Allah through this sickness, you will be cleansed and purified.
N.B. You will be cleansed and purified means that you are merely undergoing a cleansing process. Spiritually, you are being cleansed from your sins, and physically, your body is being cleansed from toxins. Hence, when this sickness has come to you from Allah Ta‘ala as a mercy in disguise, then there is no need for you to worry and grieve.
عن ابن عباس رضي الله عنهما: أن النبي صلى الله عليه وسلم دخل على أعرابي يعوده، قال: وكان النبي صلى الله عليه وسلم إذا دخل على مريض يعوده فقال له: لا بأس، طهور إن شاء الله (صحيح البخاري، الرقم: 5656)
Hazrat Ibnu Abbaas (radhiyallahu anhuma) reports that Rasulullah (sallallahu ‘alaihi wasallam) once visited a sick villager, and whenever he would visit the sick, he used to recite the following dua:
لاَ بَأسَ طَهُورٌ إنْ شَاءَ اللهُ