Musnad Ahmad 25:300

Abu Mu'awiyah narrated to us, Hajjaj narrated to us, from al-Minhal ibn 'Amr, from 'Abdullah ibn al-Harith, from Ibn 'Abbas—Abu Mu'awiyah said: I believe he attributed it to the Prophet—he said: "Whoever visits a sick person and says: 'I ask Allah the Almighty, Lord of the Mighty Throne, to heal you' (As'alu Allaha al-'Azim, Rabba al-'Arshi al-'Azim, an yashfiyaka), seven times, Allah will heal him, if his time has not yet been delayed," meaning in his appointed term. 'Abdullah said: My father said: And Yazid narrated it to us, and he did not doubt its attribution to the Prophet, and he agreed with him on the chain of narration.