It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that the door of the Prophet (peace and blessings of Allah be upon him) used to be knocked with fingernails.
Hadith Referenceالادب المفرد / كتاب الاستئذان / 1080
Hadith Gradingالألبانی:صحيح
Hadith Takhrij«صحيح : أخرجه البخاري فى التاريخ الكبير : 228/1 و البيهقي فى شعب الإيمان : 1530 و المزي فى تهذيب الكمال : 350/26 - أنظر الصحيحة : 2092»
Explanation & Benefits
Shaykh Maulana Usman Muneeb
Benefits and Issues: ➊ The purpose of knocking on the door is to inform the inhabitants of the house, not to frighten or alarm them. Therefore, one should knock on the door gently. Continuously ringing the bell or knocking loudly on the door is contrary to proper etiquette, especially in the case of a respected personality, whose honor and respect demand that the door not be struck forcefully.
➋ Knocking on the door is also akin to seeking permission; therefore, after knocking three times, one should wait. If there is no response, one should return.
➌ If the courtyard of the house is large and the gate is far from the living quarters, then there is no harm in knocking somewhat loudly.
Source: Fadlullah al-Ahad: Urdu Commentary on al-Adab al-Mufrad, Page: 1080