Al-Kāfi

Hadith 429
fasting
2ـ عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنِ ابْنِ أَبِي عُمَيْرٍ عَنْ حَمَّادٍ عَنِ الْحَلَبِيِّ عَنْ أَبِي عَبْدِ الله (عَلَيْهِ السَّلاَم) قَالَ سَأَلْتُهُ عَنِ الصَّائِمِ يَسْتَاكُ بِالْمَاءِ قَالَ لا بَأْسَ بِهِ وَقَالَ لا يَسْتَاكُ بِسِوَاكٍ رَطْبٍ.
2. Ali ibn Ibrahim has narrated from his father from ibn abu ‘Umayr from Hammad from al-Halabiy who has said the following: “I once asked abu ‘Abdillah (a.s.) about a fasting man who brushes his teeth with water. He (the Imam) said, ‘It is not harmful.‘ He added: ‘One must not brush with a wet twig.’”
Source: Shaykh Muḥammad b. Yaʿqūb al-Kulaynī