Al-Husayn ibn Said, from Hammad, from Abdullah ibn al-Mughirah, from Ibn Sinan, from Abu Abdillah عليه السلام:
Imam عليه السلام said: “The fasting person may use the tooth-stick (siwak) at whichever hour of the day he wishes.”
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 4, p. 332