Abu Hurairah (May Allah be pleased with him) reported:
The Prophet (ﷺ) said, "When Allah created Adam (ﷺ), 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 (ﷺ) 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].
App reference: Book 6, Hadith 2
Arabic/English book reference: Book 6, Hadith 846