Jami` at-Tirmidhi 725
جامع الترمذي#725Da'ifAbdullah bin Amir bin Rabi'ah narrated from his father who said:"I saw the Prophet - (a number of times) such that I was not able to count - using the Siwak while he was fasting
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Ahmad Muhammad ShakirDaif
Al-AlbaniDaif
Zubair Ali ZaiDaif
In-book reference: 8:44