Riyad as-Salihin #166

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 peace be upon you).' They replied: 'As-Salamu 'Alaikum wa Rahmatullah (May peace be upon you and the Mercy of Allah).' Thus adding in reply to him: 'wa Rahmatullah (and the Mercy of Allah)' to his greeting.".