Authentic Supplication Number 231.

Allah the exalted said: "It is from Sulayman, and it is in the name of Allah The Ever Merciful, the Mercy Bestowing" (236).

'Aba Sufyan said that the letter written to Hercules by the Messenger of Allah  contained the following: Bismillahir-rahmanir-raheemi. From Muhammad the slave, and messenger of Allah, to Hercules the great of Rome. As-Salamu `ala men 'ittaba`al-huda, 'amma ba`d (237).

Footnotes.
(236) Surat 'An-Naml (#27) V.30.
(237) In the name of Allah the Ever Merciful, Mercy Bestowing. From Muhammad the slave, and Messenger of Allah, to Hercules the great of Rome, Peace be upon him who has followed the guidance, as for what follows...

Reporters.
'Al-`Adawee said it was sound, and that it was reported by Al-Bukhari, page 14 of volume 11 of Fath Al-Baree; Muslim, page 103 volume 12; At-Tirmithi, page 500 of volume 7; and that At-Tirmithi said it was comely-sound.