It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever visits a sick person and says: 'I ask Allah the Mighty, Lord of the Mighty Throne, to cure you,' then, if his appointed time of death has not come, he will certainly recover."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12678