بُلُوغُ الْمَرَامِ
Bulugh al-Maram 350
The Book of Prayer
وَعَنْ اَلْبَرَاءِ بْنِ عَازِبٍ -رَضِيَ اَللَّهُ عَنْهُمَا- { أَنَّ اَلنَّبِيَّ - صلى الله عليه وسلم -بَعَثَ عَلِيًّا إِلَى اَلْيَمَنِ - فَذَكَرَ اَلْحَدِيثَ - قَالَ : فَكَتَبَ عَلِيٌّ - رضى الله عنه - بِإِسْلَامِهِمْ , فَلَمَّا قَرَأَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -اَلْكِتَابَ خَرَّ سَاجِدًا } رَوَاهُ اَلْبَيْهَقِيُّ . [1]
Narrated 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].
Sunnah.com reference: Book 2, Hadith 251