Dua and Dhikr
When visiting the sick.
1.
أَسْأَلُ اللَّهَ الْعَظِيمَ رَبَّ الْعَرْشِ الْعَظِيمِ أَنْ يَشْفِيَكَ
As'alullah Al-'Azeem Rabbal 'Arshil 'Azeem an yashfik
(I ask Allah the Magnificent, Lord of the Magnificent Throne to cure you)
Ibn 'Abbas narrated that the Prophet (S.A.W) said:
"There is no Muslim worshiper who visits one who is ill - other than at the time of death - and he says seven times: As'alullah Al-'Azeem Rabbal 'Arshil 'Azeem an yashfik ('I ask Allah the Magnificent, Lord of the Magnificent Throne to cure you') except when he will be cured."
(Jami` at-Tirmidhi 2083)
2.
لَا بَأْسَ طَهُورٌ إِنْ شَاءَ اللَّهُ
Laa ba'sa tahoorun 'inshaa'Allaah.
(Do not worry, it will be a purification (for you), Allah willing.)
Narrated Ibn `Abbas:
Allah's Messenger (ﷺ) entered upon sick man to pay him a visit, and said to him, "Don't worry, Allah willing, (your sickness will be) an expiation for your sins." The man said, "No, it is but a fever that is boiling within an old man and will send him to his grave." On that, the Prophet (ﷺ) said, "Then yes, it is so."
(Sahih al-Bukhari 5662, Sahih al-Bukhari 5656)