Sahih al-Bukhari, Hadith 244 ⓘ
Narrated Abu Burda: My father said, "I came to the Prophet and saw him carrying a Siwak in his hand and cleansing his teeth, saying, 'U' U'," as if he was retching while the Siwak was in his mouth."
حَدَّثَنَا أَبُو النُّعْمَانِ، قَالَ حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، عَنْ غَيْلاَنَ بْنِ جَرِيرٍ، عَنْ أَبِي بُرْدَةَ، عَنْ أَبِيهِ، قَالَ أَتَيْتُ النَّبِيَّ صلى الله عليه وسلم فَوَجَدْتُهُ يَسْتَنُّ بِسِوَاكٍ بِيَدِهِ يَقُولُ " أُعْ أُعْ " ، وَالسِّوَاكُ فِي فِيهِ، كَأَنَّهُ يَتَهَوَّعُ.
also referenced under: Book 4, Hadith 111 (In-Book) & Volume 1, Book 4, Hadith 245 (USC-MSA)