Sunan an-Nasai Book 1, Hadith 158

أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ الأَعْلَى، قَالَ حَدَّثَنَا خَالِدٌ، حَدَّثَنَا شُعْبَةُ، عَنْ عَاصِمٍ، أَنَّهُ سَمِعَ زِرَّ بْنَ حُبَيْشٍ، يُحَدِّثُ قَالَ أَتَيْتُ رَجُلاً يُدْعَى صَفْوَانَ بْنَ عَسَّالٍ فَقَعَدْتُ عَلَى بَابِهِ فَخَرَجَ فَقَالَ مَا شَأْنُكَ قُلْتُ أَطْلُبُ الْعِلْمَ ‏.‏ قَالَ إِنَّ الْمَلاَئِكَةَ تَضَعُ أَجْنِحَتَهَا لِطَالِبِ الْعِلْمِ رِضًا بِمَا يَطْلُبُ ‏.‏ فَقَالَ عَنْ أَىِّ شَىْءٍ تَسْأَلُ قُلْتُ عَنِ الْخُفَّيْنِ ‏.‏ قَالَ كُنَّا إِذَا كُنَّا مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم فِي سَفَرٍ أَمَرَنَا أَنْ لاَ نَنْزِعَهُ ثَلاَثًا إِلاَّ مِنْ جَنَابَةٍ وَلَكِنْ مِنْ غَائِطٍ وَبَوْلٍ وَنَوْمٍ ‏.‏

Zirr bin Hubaish narrated:"I came to a man called Safwan bin 'Assal and sat at his door. He came out and said: 'What do you want?' I said: 'I am seeking knowledge.' He said: 'The angels lower their wings for the seeker of knowledge out of pleasure at what he is seeking.' He said: 'What do you want to know about?' I said: 'About the Khuffs.' He said: 'When we were with the Messenger of Allah (ﷺ) on a journey, he told us not to take them off for three days except in the case of Janabah, but not in the case of defecation, urinating or sleep