Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrated that the Prophet (peace and blessings be upon him) said: "Whoever visits a sick person whose appointed time of death has not yet come, and says seven times in his presence: 'I ask Allah the Mighty, Lord of the Mighty Throne, to cure you,' he will surely be cured."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12220