Surah An-Nisah, ayat 86:
Dan apabila kamu diberikan penghormatan dengan sesuatu ucapan hormat (seperti memberi salam), maka balaslah penghormatan itu dengan yang lebih baik daripadanya, atau balaslah dia (dengan cara yang sama). Sesungguhnya Allah sentiasa menghitung tiap-tiap sesuatu.
When you are greeted with a greeting, greet in return with what is better than it, or (at least) return it equally. Certainly, Allâh is Ever a Careful Account Taker of all things.
How the salam (greetings) began.
Narrated Abu Hurairah r.a: The Prophet s.a.w said, "Allah created Adam in His Image, *sixty cubits (about 30 metres) in height.
When He(Allah) created him(Adam), He(Allah) said (to him-Adam),
"Go and greet that group of angels sitting there and listen what they will say in reply to you, for that will be your greeting and the greeting of your offspring."
Adam (went and) said, "As-Salamu 'Alaikum (peace be upon you)."
They replied, "As-Salamu 'alaikum wa Rahmatullah (Peace and Allah's Mercy be to you)."
So they increased "wa Rahmatullah."
The Prophet s.a.w added, "So, whoever will enter Paradise will be of the shape and picture of Adam.
Since then the creation of Adam's (offspring-that is the stature of human beings) is being diminished continuously up to the present time."
(Sahih Al-Bukhari, Vol. 8, hadith # 246).
**His Image means that Adam has been bestowed with life, knowledge, power of hearing, seeing, understanding but the features of Adam are different from those of Allah, only the names are the same, e.g. Allah has life and knowledge and power of understanding and Adam also has them but there is no comparison between the Creator and the created thing.
As Allah says in the Qur'an: "There is nothing like unto Him (Allah) and He (Allah) is the All-Hearer, the All-Seer.
(V.42:11) Allah does not eat or sleep while Adam used to eat and sleep.