Authentic Supplication Number 151.

It is reported of the Prophet  that he said: "Whoever visited a sick person whose set time [of death] has not yet come, and says at his place seven times: 'As'alul-laha 'al `atheema, rabbal `arshil `atheemi 'an yahfiyaka (170), Allah would not but heal him."

Footnotes.
(170) I ask Allah the Majestic, Lord of the Magnificent Throne, that He would heal you.

Reporters.
Reported by Abu Dawood, and At-Tirmithi, and he said it was comely. Al-Albani said that it is as he said. He also wrote, followed by an exclamation mark, that Al-Hakim said it was sound according to the conditions of Al-Bukhari, and that Ath-Thahabi agreed with him, It is #122 in The Authentic of Good Sayings.