Hadith 3:93
She also said, “The Prophet was accustomed to use the tooth-stick and then give it to me to wash it. I would first use it myself, then wash it and hand it to him.” Abu Dawud transmitted it.
وَعَنْهَا قَالَتْ: كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَسْتَاكُ فَيُعْطِينِي السِّوَاكَ لِأَغْسِلَهُ فَأَبْدَأُ بِهِ فَأَسْتَاكُ ثُمَّ أَغْسِلُهُ وَأَدْفَعُهُ إِلَيْهِ. رَوَاهُ أَبُو دَاوُد
Original Reference: Mishkat al-Masabih 384