Musnad Ahmad
It was narrated from ‘Abd Khair that ‘Ali (رضي الله عنه) said: I used to think that the bottoms of the feet were more deserving of being wiped than the tops, until I saw the Messenger of Allah (ﷺ) wiping the top (of his feet).
حَدَّثَنَا عَبْد اللَّهِ، حَدَّثَنَا إِسْحَاقُ بْنُ إِسْمَاعِيلَ، حَدَّثَنَا وَكِيعٌ، حَدَّثَنَا الْأَعْمَشُ، عَنْ أَبِي إِسْحَاقَ، عَنْ عَبْدِ خَيْرٍ، عَنْ عَلِيٍّ، رَضِيَ اللَّهُ عَنْهُ قَالَ كُنْتُ أَرَى أَنَّ بَاطِنَ الْقَدَمَيْنِ أَحَقُّ بِالْمَسْحِ مِنْ ظَاهِرِهِمَا حَتَّى رَأَيْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَمْسَحُ ظَاهِرَهُمَا.
Grade: Sahih because of corroborating evidence] (Darussalam)
Reference