Riyad as-Salihin - The Book of Greetings - كتاب السلام

وعن أبي هريرة رضي الله عنه عن البني صلى الله عليه وسلم قال‏:‏ لما خلق الله تعالى آدم عليه السلام قال‏:‏ اذهب فسلم على أولئك -نفر من الملائكة جلوس- فاستمع ما يحيونك، فإنه تحيتك وتحية ذريتك‏.‏ فقال‏:‏ السلام عليكم فقالوا‏:‏ السلام عليك ورحمة الله، فزادوه‏:‏ ورحمة الله‏"‏ ‏(‏‏(‏متفق عليه‏)‏‏)‏‏.‏

Abu Hurairah (May Allah be pleased with him) reported:
The Prophet () said, "When Allah created Adam (), He said to him: 'Go and greet that company of angels who are sitting there - and then listen to what they are going to say in reply to your greetings because that will be your greeting and your off-spring's.' Adam () said to the angels: 'As-Salamu 'Alaikum (may you be safe from evil).' They replied: 'As-Salamu 'Alaikum wa Rahmatullah (may you be safe from evil, and Mercy of Allah be upon you).' Thus adding in reply to him: 'wa Rahmatullah (and Mercy of Allah)' to his greeting."

[Al- Bukhari and Muslim].