Funerals (Kitab Al-Jana'iz)
Dawud :: Book 20 : Hadith 3100
Narrated Abdullah ibn Abbas:
The Prophet (peace_be_upon_him) said: If anyone visits a sick whose time (of death) has not come, and says with him seven times: I ask Allah, the Mighty, the Lord of the mighty Throne, to cure you, Allah will cure him from that disease.