Hadith Q193

إِذْ هُوَ لَابِسٌ أَحَدَ الْخُفَّيْنِ قَبْلَ كَمَالِ الطَّهَارَةِ، وَالنَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِنَّمَا رَخَّصَ فِي الْمَسْحِ عَلَى الْخُفَّيْنِ إِذَا لَبِسَهُمَا عَلَى طَهَارَةٍ، وَمَنْ ذَكَرْنَا فِي هَذَا الْبَابِ صِفَتَهُ هُوَ لَابِسٌ أَحَدَ الْخُفَّيْنِ عَلَى غَيْرِ طُهْرٍ، إِذْ هُوَ غَاسِلٌ إِحْدَى الرِّجْلَيْنِ لَا كِلْتَيْهِمَا عِنْدَ لُبْسِهِ أَحَدَ الْخُفَّيْنِ‏.‏
Because he wore one sock before completing purification, and the Prophet Muhammad (peace be upon him) granted concession to wipe over the socks when a person has worn them in a state of purity. In this chapter, the condition of the person we have described is that he wore one sock without completing ablution, because when he wore one sock, he had washed one foot, not both feet.
Hadith Reference صحيح ابن خزيمه / جماع أبواب المسح على الخفين / Q193