Is there a Hadith that necessitates bidding farewell with the Salaam, given that it is commonly known to initiate a greeting with the Salaam upon meeting?

It is sunnah (definition) for a man, when he arrives at a gathering, to greet its occupants with salutations (salam). And when he wishes to leave and depart, he should greet them again, because the Prophet, peace and blessings be upon him, said: "When one of you arrives at a gathering, let him offer salam. And when he wishes to depart, let him offer salam. The first is no more deserving than the last." So, offering salam upon departure is sunnah, just as offering salam upon arrival is. Both are supplications for which a response is recommended.