Sunan an-Nasa'i · Book 1 · Hadith 3
أَخْبَرَنَا أَحْمَدُ بْنُ عَبْدَةَ، قَالَ حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، قَالَ أَخْبَرَنَا غَيْلاَنُ بْنُ جَرِيرٍ، عَنْ أَبِي بُرْدَةَ، عَنْ أَبِي مُوسَى، قَالَ دَخَلْتُ عَلَى رَسُولِ اللَّهِ صلى الله عليه وسلم وَهُوَ يَسْتَنُّ وَطَرَفُ السِّوَاكِ عَلَى لِسَانِهِ وَهُوَ يَقُولُ " عَأْعَأْ " .
It was narrated that Abu Musa said: "I came to the Messenger of Allah (ﷺ) when he was using the Siwak, and the end of the Siwak was on his toungue, and he was saying, "'A','a'."
Sahih