It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him), he said: When someone enters and does not say «السلام علیکم», then say to him: No, not until you bring the key, that is, say «السلام علیکم».
Hadith Referenceالادب المفرد / كتاب الاستئذان / 1067
Hadith Gradingالألبانی:صحيح
Hadith Takhrij«صحيح : أخرجه الخطيب فى الجامع لأخلاق الراوي : 226»
Explanation & Benefits
Shaykh Maulana Usman Muneeb
Benefits and Issues: The etiquette of going to someone’s house is that first one should say "as-salamu alaykum" and then seek permission. Seeking permission before greeting is not correct, nor does seeking permission take the place of the greeting.
Source: Fadlullah al-Ahad: Urdu Commentary on al-Adab al-Mufrad, Page: 1067