Reference : Hisn al-Muslim 148

أَسْـأَلُ اللهَ العَـظيـم، رَبَّ العَـرْشِ العَـظيـم أَنْ يَشْفـيك .
(سبع مرات)
As'alullāha ‘l-`Aẓīma Rabba ‘l-`Arshil-`Aẓīmi an yashfiyak.

I ask Almighty Allah, Lord of the Magnificent Throne, to make you well. 
(Recite seven times in Arabic .)

Reference:
At-Tirmidhi, Abu Dawud. See also Al-Albani, Sahih At-Tirmidhi 2/210 and 
Sahihul-Jami' As-Saghir 5/180.

Chapter: When visiting the sick