Hadith: A man came to the Prophet (may Allah's peace and blessings be upon him) and said: Assalāmu ‘alaykum. The Prophet responded to his greeting and the man sat down. The Prophet (may Allah's peace and blessings be upon him) said: Ten. Then another man came and said: Assalāmu ‘alaykum wa rahmatullah. The Prophet replied to his greeting and the man sat down. The Prophet (may Allah's peace and blessings be upon him) said: Twenty

عن عمران بن الحُصَيْن رضي الله عنهما ، قال: جاء رجل إلى النبي صلى الله عليه وسلم فقال: السلام عليكم، فَرَدَّ عليه ثم جَلس، فقال النبي صلى الله عليه وسلم : «عَشْرٌ» ثم جاء آخر، فقال: السلام عليكم ورحمة الله، فَرَدَّ عليه فجلس، فقال: «عشرون» ثم جاء آخر، فقال: السلام عليكم ورحمة الله وبركاته، فَرَدَّ عليه فجلس، فقال: «ثلاثون».
[حسن] - [رواه أبو داود والترمذي وأحمد والدارمي]

‘Imrān ibn al-Husayn (may Allah be pleased with him) reported: A man came to the Prophet (may Allah's peace and blessings be upon him) and said: "Assalāmu ‘alaykum (Peace be upon you)." The Prophet responded to his greeting and the man sat down. The Prophet (may Allah's peace and blessings be upon him) said: ‘Ten.’ Then another man came and said: "Assalāmu ‘alaykum wa rahmatullāh (May the peace and mercy of Allah be upon you)." The Prophet replied to his greeting and the man sat down. Then another man came and said: "Assalāmu ‘alaykum wa rahmatullāh wa barakātuh (May the peace, mercy, and blessings of Allah be upon you)." The Prophet replied to his greeting and the man sat down. The Prophet (may Allah's peace and blessings be upon him) said: ‘Thirty.'"
Hasan/Sound. - [At-Tirmidhi]

Explanation

A man came to the Prophet (may Allah's peace and blessings be upon him) and said: "Assalāmu ‘alaykum (Peace be upon you)." The Prophet responded to him and the man sat down. Then the Prophet (may Allah's peace and blessings be upon him) said that that man had obtained ten good deeds. This is the reward earned by anyone says this phrase of the greeting of peace. And Allah may still multiply this if He so wills. Then, someone else came and said: "Assalāmu ‘alaykum wa rahmatullāh (May the peace and mercy of Allah be upon you)." The Prophet responded to his greeting and the man sat down. Then the Prophet (may Allah's peace and blessings be upon him) said that this man had gained twenty good deeds, as his greeting was longer than the previous one by saying: "wa rahmatullah (and the mercy of Allah)". Then, a third man came and said: "Assalāmu ‘alaykum wa rahmatullāh wa barakātuh (May the peace, mercy, and blessings of Allah be upon you)." The Prophet (may Allah's peace and blessings be upon him) responded to him and the man sat down. Then, the Prophet (may Allah's peace and blessings be upon him) said that he had gained thirty good deeds. This last one is the most complete phrase of the Islamic greeting of peace.