(501) بَابُ إِذَا دَخَلَ وَلَمْ يَسْتَأْذِنْ
Abu Hurayra reported that the Messenger of Allah, may Allah blesshim and grant him peace, said, "If someone looks inside, he should notbe given permission to enter."
Reference: Al-Adab Al-Mufrad 1082
In-book reference: Book 43, Hadith 32
English translation: Book 43, Hadith 1082