I heard Ayesha report that the Prophet ﷺ said, "The siwak is a means of purifying the mouth and pleasing the Lord."
أخبرنا حميد بن مسعدة ومحمد بن عبد الأعلى عن يزيد وهو ابن زريع قال
سمعت عائشة عن النبي ﷺ قال السواك مطهرة للفم مرضاة للرب
Sahih. It was also narrated by Ahmad 6/124 from the hadith of Yazid bin Zuray with the same wording, and Darawardi followed him in agreement, as recorded by Abu Yaala 8/315, hadith (4916), with a hasan chain. It is also in Kubra, hadith (4), and Bukhari mentioned it without a chain in the Book of Fasting, chapter on using the tooth-stick, fresh or dry, for one who is fasting, before hadith (1934). The hadith has many corroborating narrations found in Ibn Khuzayma, hadith (135), Ahmad, and others.