The Messenger of Allaah -sallAllaahu alayhi wa sallam-said:
Who ever visits a sick person who has not reached his appointed time, then says for him seven times: –
أَسْأَلُ اللَّهَ الْعَظِيمَ رَبَّ الْعَرْشِ الْعَظِيمِ أَنْ يَشْفِيَكَ
I ask Allaah The ‘Atheem (The Supreme), Lord of the magnificent ‘Arsh to cure you -Except that Allaah cures him.
As’alul-laha ‘al-atheema, rabbal ‘arshil átheemi an yashfiyaka
At-Tirmidhi, Abu Dawud. See also Al-Albaani, Sahih At-Tirmidhi 2/210 and Sahihul-Jaami As-Saghir 5/180