The Prophet (may Allah's peace and blessings be upon him) sent Khālid ibn al-Walīd to the people of Yemen to invite them to Islam, but they did not respond to him. Then, the Prophet sent ‘Ali ibn Abi Tālib
Good hadithعن البراء رضي الله عنه قال: بعث النبي صلى الله عليه وسلم خالد بن الوليد إلى أهل اليمن يَدْعُوهُمْ إلى الإسلام فَلَمْ يُجِيبُوهُ، ثم إنَّ النبي صلى الله عليه وسلم بعث علي بن أبي طالب، وأمره أن يَقْفُلَ خالد ومن كان معه إلا رجل ممن كان مع خالد أحب أن يُعَقِّبَ مع علي رضي الله عنه فَلْيُعَقِّبْ معه قال البراء فَكُنْتُ مِمَّنْ عَقَّبَ مَعَهُ، فَلَمَّا دَنَوْنَا مِنَ القوم خرجوا إلينا فصلى بنا عليٌّ رضي الله عنه وَصَفَّنَا صَفًّا واحدا، ثم تقدَّم بين أيدينا، فقرأ عليهم كتاب رسول الله صلى الله عليه وسلم فَأَسْلَمَتْ هَمْدَانُ جَمِيعًا، فكتب علي رضي الله عنه إلى رسول الله صلى الله عليه وسلم بإسلامهم، فلمَّا قرأ رسول الله صلى الله عليه وسلم الكتاب خَرَّ ساجدا، ثم رفع رأسه، فقال: السَّلَامُ عَلَى هَمْدَانَ، السلامُ على هَمْدَانَ.
Al-Barā' (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) sent Khālid ibn al-Walīd to the people of Yemen to invite them to Islam, but they did not respond to him. Then, the Prophet sent ‘Ali ibn Abi Tālib and ordered him to send back Khālid and those with him except if any of the men with Khālid wanted to stay with ‘Ali, in which case they should be allowed to do so. I was one of those who stayed with him. When we drew close to the people, they went out toward us, and ‘Ali (may Allah be pleased with him) led us in prayer and lined us in one row. Then, he stepped forward and stood before us and read out the Messenger’s letter to them. Thereupon, the entire Hamdān tribe embraced Islam. ‘Ali (may Allah be pleased with him) wrote to the Messenger of Allah (may Allah’s peace and blessings be upon him) informing him of their conversion to Islam. On reading that, the Messenger of Allah (may Allah’s peace and blessings be upon him) fell in prostration. Then, he raised his head and said: "Peace be upon Hamdān, peace be upon Hamdān."
Explanation
The Hadīth indicates that the Prophet (may Allah's peace and blessings be upon him) used to prostrate himself in gratitude to Allah, the Exalted, whenever he received good news. One of these occasions was when he sent ‘Ali (may Allah be pleased with him) to the people of Yemen to invite them to Islam after they had refused to convert to this religion when invited by Khālid ibn al-Walīd. When ‘Ali invited them, the whole of Hamdān responded and embraced Islam. Thereupon, ‘Ali sent a letter to the Prophet (may Allah's peace and blessings be upon him) wherein he conveyed this good news to him. Upon that, the Prophet (may Allah's peace and blessings be upon him) prostrated himself in gratitude to Allah, the Exalted.
Source · Al-Bayhaqi
From the Hadith Encyclopedia (hadeethenc.com).