Hadith: When the people of Yemen came, the Messenger of Allah (may Allah's peace and blessings be upon him) said: There have come to you the people of Yemen

عن أنس رضي الله عنه قَالَ: لَمَّا جَاءَ أهْلُ اليَمَنِ، قَالَ رسولُ الله صلى الله عليه وسلم : "قَدْ جَاءكُمْ أهْلُ اليَمَنِ" وَهُمْ أوَّلُ مَنْ جَاءَ بِالمُصَافَحَة.
[صحيح] - [رواه أبو داود وأحمد]

Anas (may Allah be pleased with him) reported: When the people of Yemen came, the Messenger of Allah (may Allah’s peace and blessings be upon him) said: "There have come to you the people of Yemen." They were the first to bring the (practice of) handshake.
[Authentic hadith] - [Narrated by Abu Daoud - Narrated by Ahmad]

Explanation

The Prophet (may Allah's peace and blessings be upon him) said: "There have come to you the people of Yemen,” he meant by this the Ash‘ariyyūn, who were the people of Abu Mūsa al-Ash‘ari (may Allah be pleased with them). This is supported by the Hadīth narrated by Ahmad in his Musnad (3/155) on the authority of Anas ibn Mālik who reported: The Messenger of Allah (may Allah’s peace and blessings be upon him) said: "Tomorrow a people will come to you whose hearts are softer for Islam than yours." Anas said: "The Ash‘ariyyūn came, and among them was Abu Mūsa al-Ash‘ari. When they approached Madīnah, they began chanting lines of poetry that meant: 'Tomorrow we will meet our beloved ones, Muhammad and his group.' When they arrived, they began shaking hands. So, they were the first to introduce the handshake.”