(5) باب المسح على الخفين
Narrated Safwan bin 'Assal (rad):
When we were on a journey, the Prophet (ﷺ) used to command us to wear our leather socks for three days and three nights, whether we had to answer the call of nature or slept. However, in case of ejaculation or sexual impurity, he commanded us to remove the leather socks. [Reported by An-Nasa'i and At-Tirmidhi, version is of the latter. With them Ibn Khuzaima graded it Sahih (sound)].
Reference: Bulugh al-Maram 61
In-book reference: Book 1, Hadith 71
English translation: Book 1, Hadith 66