Kildah bin Al-Hanbal (May Allah be pleased with him) reported:
I visited the Prophet (ﷺ) and I entered his house without seeking permission. So he said, "Go back and say: 'As-Salamu 'alaikum (may you be safe from evil). May I come in?"'[Abu Dawud and At- Tirmidhi].
App reference: Book 6, Hadith 29
Arabic/English book reference: Book 6, Hadith 873