And from him, from al-Qasim ibn Muhammad, from Ali, from Abu Basir, and Muhammad ibn Abi Umayr, from Hammad, from al-Halabi, all together from Abu Abdillah عليه السلام, peace be upon him:
Imam عليه السلام said: “The fasting person may use the tooth-stick (siwak) at whichever part of the day he wishes.”
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 4, p. 332