It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him), he says that when a person says: May I come in? and does not say «السلام علیکم», then say to him: No, you cannot come in until you bring the key. I said: «السلام» ......? He said: Yes.
Hadith Referenceالادب المفرد / كتاب الاستئذان / 1083