The Messenger of Allah ﷺ said, "The siwak is a means of purifying the mouth and pleasing the Lord."
حدثنا إسماعيل عن محمد بن إسحاق قال حدثني عبد الله بن محمد بن عبد الرحمن بن أبي بكر
قال رسول الله ﷺ السواك مطهرة للفم مرضاة للرب
Its isnad is sahih. Abdullah bin Muhammad bin Abdul-Rahman bin Abu Bakr is reliable, and his hadith appears in the Sahihayn. Ayesha was his father's paternal aunt. The hadith was narrated by Bukhari, in suspended form, in 4/158 before number 1934, Fath, in the book of fasting. Nasai narrated it in 1/10, number 5, Ibn Majah in 1/105, number 286, and Darimi in 1/184, number 681, all of them in the book of purification.