Hadithcore
Al-Adab Al-Mufrad, 1090
sahih· Al-Albani

Muslim ibn Nadhir said, "A man asked permission to enter from Hudhayfa. He looked in and said, 'May I come in?' Hudhayfa replied, 'As for you eye, it has already entered. As for your bottom, it has not entered.'"

حَدَّثَنَا مُحَمَّدُ بْنُ كَثِيرٍ، قَالَ‏:‏ أَخْبَرَنَا سُفْيَانُ، عَنْ أَبِي إِسْحَاقَ، عَنْ مُسْلِمِ بْنِ نَذِيرٍ قَالَ‏:‏ اسْتَأْذَنَ رَجُلٌ عَلَى حُذَيْفَةَ فَاطَّلَعَ وَقَالَ‏:‏ أَدْخُلُ‏؟‏ قَالَ حُذَيْفَةُ‏:‏ أَمَّا عَيْنُكَ فَقَدْ دَخَلَتْ، وَأَمَّا اسْتُكَ فَلَمْ تَدْخُلْ‏.‏

References2 variants
In-Book Reference
Book 43, Hadith 40
English Reference
Book 43, Hadith 1090