It is narrated from Muslim bin Nazeer (may Allah have mercy on him), he said that a man sought permission from Sayyiduna Hudhaifah (may Allah be pleased with him), so he peeked in and said: "May I come in?" Sayyiduna Hudhaifah (may Allah be pleased with him) said: "Your eye has already entered, only your body remains (so what is the need for permission now?)"
And a man asked: "Should I seek permission even to go to my mother?" He replied: "If you do not seek permission, you may see something that you would dislike to see."
Hadith Referenceالادب المفرد / كتاب الاستئذان / 1090
Hadith Gradingالألبانی:صحيح ، حسن
Hadith Takhrij«صحيح ، حسن : أخرجه ابن أبى شيبة : 26237 و الخرائطي فى اعتلال القلوب : 281 و أخرجه عبدالرزاق : 19421 و البيهقي فى الكبرىٰ : 97/7»