Mishkat al-Masabih 3 Purification كتاب الطهارة 113

وَعَنْ أَنَسٍ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا تَوَضَّأَ أَخَذَ كَفًّا مِنْ مَاءٍ فَأَدْخَلَهُ تَحْتَ حَنَكِهِ فَخَلَّلَ بِهِ لحيته وَقَالَ: «هَكَذَا أَمرنِي رَبِّي» . رَوَاهُ أَبُو دَاوُد

Anas said that when God’s messenger performed ablution he took a handful of water, and putting it under his chin, made it go through his beard, saying, “Thus did my Lord command me.” Abu Dawud transmitted it.