What is the ruling on initiating and returning the greeting of peace (Salam)? Is it a Sunnah, Fard Kifayah, or Fard Ayn?

Initiating the greeting (salam) is a Sunnah according to the majority of scholars. It is an individual Sunnah (Sunnah 'ayn) for a single person and a communal Sunnah (Sunnah kifayah) for a group. It is better for everyone in the group to give the greeting to earn the reward. Replying to the greeting, however, is obligatory by consensus. If the greeting is directed at one person, the reply is an individual obligation (fard 'ayn). If it is directed at a group, it is a communal obligation (fard kifayah). It is a condition for both initiating and replying to the greeting that the voice be raised sufficiently for it to be heard. The desirability of initiating the greeting is indicated by the Prophet's (peace be upon him) saying: "Spread peace among yourselves." The obligation of replying is indicated by Allah's (the Exalted) saying: "And when you are greeted with a greeting, greet in return with one better than it, or [at least] return it [in a similar manner]."