I came in upon the Messenger of Allah ﷺ while he was using the tooth-stick. He had placed the end of the stick on his tongue and was brushing upward. Hammad described it as if he were raising his tooth-stick.Hammad said, Ghaylan described it for us, saying, "He used to brush lengthwise."
حدثنا يونس بن محمد قال ثنا حماد بن زيد حدثنا غيلان بن جرير عن أبي بردة
دخلت على رسول الله ﷺ وهو يستاك وهو واضع طرف السواك على لسانه يستن إلى فوق فوصف حماد كأنه يرفع سواكه قال حماد ووصفه لنا غيلان قال كان يستن طولا
Its isnad is sahih, and its narrators are trustworthy and well known. The hadith was narrated by Muslim, 1/220, no. 254, in the chapter on Purification, Using the Tooth-Stick, and by Abu Dawud, 1/42, no. 49, with similar wording, and by Nasai, 1/9, with similar wording.