Sahih Muslim Book 1, Hadith Number 63.

Chapter : Concerning the eminence of Islam and of the affairs which are excellent.

It is narrated on the authority of ‘Abdullah b. ‘Amr that a man asked the Messenger of Allah (may peace and blessings be upon him) which of the merits (is superior) in Islam. He (the Holy Prophet) remarked: That you provide food and extend greetings to one whom you know or do not know.