Musnad Ahmad 25:344

Hashim ibn al-Qasim narrated to us, Shu'bah narrated to us, from Abu Khalid Yazid, from al-Minhal ibn 'Amr, from Sa'id ibn Jubayr, from Ibn Abbas, from the Prophet (peace and blessings of Allah be upon him), who said: "Whoever visits a sick person whose time of death has not yet arrived, and says seven times: 'I ask Allah the Almighty, Lord of the Magnificent Throne, to heal him,' he will be cured."