"We said: 'O Messenger of Allah(ﷺ), (we know) this (greeting of) Salam, but what does seeking permission to enter mean?' He said: 'It means a man saying SubhanAllah, and Allahu Akbar and Al Hamdulillah, and clearing his throat, announcing his arrival to the people in the house."
Sunan Ibn Majah Book 33, Hadith 51
← Previous Hadith "Why did you go back?" He said: "I asked permission to enter three times, as the Messenger of Allah(ﷺ) enjoined upon us, then if we are given permission we should enter, otherwise we should go back." He said: "You should bring me proof of that, or else!" Then he came to a gathering of his people and asked them to swear by Allah concerning that, and they did so, so he let him go."

Sunan Ibn Majah Book 33, Hadith 50 Next Hadith → "I had two times of visiting the Messenger of Allah(ﷺ), at night and during the day. If I came to him when he was praying, he would clear his throat (to let me know he was praying)."

Sunan Ibn Majah Book 33, Hadith 52