Mishkat al-Masabih 3 Purification كتاب الطهارة 112

وَعَن الْمُسْتَوْرد بن شَدَّاد قَالَ: رَأَيْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا تَوَضَّأَ يُدَلِّكُ أَصَابِعَ رِجْلَيْهِ بِخِنْصَرِهِ. رَوَاهُ التِّرْمِذِيّ وَأَبُو دَاوُد وَابْن مَاجَه

Al-Mustaurid b. Shaddad said that he saw God’s messenger rubbing his toes with his little finger when he performed ablution. Tirmidhi, Abu Dawud and Ibn Majah transmitted it.