One should recite the following dua for the sick person:
يَا حَلِيمُ يَا كَرِيمُ اشْفِ فُلَانًا
O the most tolerant, O the most noble, cure (in place of the word ‘fulaan’, mention the name of the sick person)
عن فضيل بن عمرو، قال: جاء رجل إلى علي فقال: إن فلانا شاك، قال: فيسرك إن يبرأ؟ قال: نعم، قال: قل: يا حليم، يا كريم، اشف فلانا
(المصنف لابن أبي شيبة، الرقم 24048)
Hazrat Fuzail bin Amr (rahimahullah) reports that a person once came to Hazrat Ali (radhiyallahu ‘anhu) and said, “So-and-so is feeling sick.” Hazrat Ali (radhiyallahu ‘anhu) asked, “Will it please you that he recovers from his illness?” The person replied in the affirmative. Hazrat Ali (radhiyallahu ‘anhu) advised him to recite the following dua:
يَا حَلِيمُ يَا كَرِيمُ اشْفِ فُلَانًا
Source:Ihyaauddeen