The Book of Clothes and Adornment
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " لاَ يَمْشِ أَحَدُكُمْ فِي نَعْلٍ وَاحِدَةٍ لِيُنْعِلْهُمَا جَمِيعًا أَوْ لِيَخْلَعْهُمَا جَمِيعًا " .Abu Huraira reported Allah's Messenger (ﷺ) as saying:None of you should walk in one sandal; either he should wear the two or should take off the two
From Sahih Muslim, whose reports the scholars accept as authentic.