Al-Bara' bin 'Azib (RA):
The Prophet (ﷺ) sent 'Ali to Yemen. The narrator mentioned the complete Hadith. He said, "Ali sent a letter regarding their (the people of Yemen) acceptance of Islam. When Allah's Messenger (ﷺ) read the letter, he prostrated in gratitude to Allah the Most High for that."
[Reported by al-Baihaqi].