Hadithcore
Riyad as-Salihin, 845

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." <b>[Al- Bukhari and Muslim]</b>.

وعن أبي هريرة رضي الله عنه عن النبي 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)‏‏)‏‏.‏

References1 variant
In-Book Reference
Book 5, Hadith 2
Sharh · explanationclick to expand
<b>Guidance from the Hadith:</b><br> 1) The Islamic greeting is taken from the honorable angels by the command of Allah Almighty.<br> 2) The best form for returning the greeting is: “Wa ‘alykum as-salām wa rahmatullāh wa barakātuh” (And peace be upon you and the mercy of Allah and His blessings).<br> Note:<br> It is not permissible to replace the greeting reported in the Shariah with phrases that people commonly say, such as “good morning” or “welcome”. This would be regarded as trading what is superior for what is inferior. Likewise, we may only return the greeting with the form reported in the Shariah. Such phrases as “and upon you be what you have said” and “hello and welcome” are not Islamic responses to the greeting.
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