It is narrated from Hazrat Anas (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Such people will come to you whose hearts will be softer than yours." Accordingly, once the Ash'aris came, among whom was Hazrat Abu Musa Ash'ari (may Allah be pleased with him) as well. When they reached near Madinah Munawwarah, they began to recite these rajaz verses: "Tomorrow we will meet our friends, that is, Muhammad (peace and blessings be upon him) and his companions." And these were the first people who established the practice of shaking hands.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 13334
Hadith Gradingحکم دارالسلام:حديث صحيح، وهذا إسناد حسن.