Narrated Abu Huraira:

The Prophet ﷺ said, "If somebody eats or drinks forgetfully then he should complete his fast, for what he has eaten or drunk, has been given to him by Allah." Narrated 'Amir bin Rabi'a, "I saw the Prophet cleaning his teeth with Siwak while he was fasting so many times as I can't count." And narrated Abu Huraira, "The Prophet ﷺ said, 'But for my fear that it would be hard for my followers, I would have ordered them to clean their teeth with Siwak on every performance of ablution." The same is narrated by Jabir and Zaid bin Khalid from the Prophet who did not differentiate between a fasting and a nonfasting person in this respect (using Siwak). Aisha said, "The Prophet ﷺ said, "It (i.e. Siwak) is a purification for the mouth and it is a way of seeking Allah's pleasures." Ata' and Qatada said, "There is no harm in swallowing the resultant saliva."

حَدَّثَنَا عَبْدَانُ، أَخْبَرَنَا يَزِيدُ بْنُ زُرَيْعٍ، حَدَّثَنَا هِشَامٌ، حَدَّثَنَا ابْنُ سِيرِينَ، عَنْ أَبِي هُرَيْرَةَ ـ رضى الله عنه ـ عَنِ النَّبِيِّ ﷺ قَالَ
" إِذَا نَسِيَ فَأَكَلَ وَشَرِبَ فَلْيُتِمَّ صَوْمَهُ، فَإِنَّمَا أَطْعَمَهُ اللَّهُ وَسَقَاهُ ".