I Ask Allah the Mighty to Cure You
أَسْأَلُ اللَّهَ الْعَظِيمَ رَبَّ الْعَرْشِ الْعَظِيمِ أَنْ يَشْفِيَكَ
As'alullahal-'azima rabbal-'arshil-'azimi an yashfiyak.
I ask Allah the Mighty, Lord of the mighty Throne, to cure you.
What it means
Short enough to say at a bedside without tiring the person who is ill. It names Allah by His greatness twice, which is the point: the illness is large to the patient, and this reminds everyone in the room who is larger.
When and how
Say it seven times beside the person who is ill. You may say it in their presence out loud, or for them when you are away.
Its virtue
The Prophet ﷺ said that no servant visits a sick person whose time has not yet come and says this seven times except that Allah cures him of that illness, reported in Sunan Abi Dawud 3106 and Jami at-Tirmidhi 2083.
Source
Abu Dawud 3106; At-Tirmidhi 2083sahih