Mishkat al-Masabih
Mishkat al-Masabih 522
Arabic hadith text
وَعنهُ قَالَ: رَأَيْتُ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يمسح على الْخُفَّيْنِ على ظاهرهما. رَوَاهُ التِّرْمِذِيّ وَأَبُو دَاوُد
Translation
He also said: ‘I saw the Prophet wiping over the outer surfaces of his khuffs.’ Al-Tirmidhī and Abū Dāwūd transmitted it.