Sahih Muslim
The Book of Faith (Kitab Al-Iman)
Book Number: 001 / Hadith Number: 63
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.