"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'." (Using translation from Nasāʾī 3)  

دَخَلْتُ عَلَى رَسُولِ اللهِ ﷺ وَهُوَ يَسْتَنُّ وَطَرَفُ السِّوَاكِ عَلَى لِسَانِهِ وَهُوَ يَقُولُ «عَأْ عَأْ»