It was narrated from ‘Imran ibn al-Husayn (may Allah be pleased with him) that he said: A man came to the Prophet (blessings and peace of Allah be upon him) and said: As-salaamu aleikum. He returned the greeting, then he sat down, and the Prophet (blessings and peace of Allah be upon him) said: “Ten [meaning ten hasanat or rewards for a good deed].” Another man came and said: As-salaamu ‘alaykum wa rahmatullah. He returned the greeting, then he sat down. The Prophet (blessings and peace of Allah be upon him) said: “Twenty.” Another man came and said: As-salaamu ‘alaykum wa rahmatullahi wa barakatuhu. He returned the greeting, then he sat down. The Prophet (blessings and peace of Allah be upon him) said: “Thirty.”
Narrated by Abu Dawud (5195) and by at-Tirmidhi (2689), who said: It is hasan. It was classed as sahih by al-Albani in Sahih at-Tirmidhi.
It was narrated that ‘A’ishah (may Allah be pleased with her) said: The Messenger of Allah (blessings and peace of Allah be upon him) said to me: “Jibril conveys greetings of salaam to you.” I said: And upon him be peace and the mercy of Allah.
Narrated by al-Bukhari (3045) and Muslim (2447).
For more information visit : https://islamqa.info/en/answers/132956/the-best-phrases-for-giving-and-returning-the-greeting-of-salaam
