From Sahih Bukhari (Hadith 4438):
Narrated Aisha:`Abdur-Rahman bin Abu Bakr entered upon the Prophet (ﷺ) while I was supporting the Prophet (ﷺ) on my chest. `AbdurRahman had a fresh Siwak then and he was cleaning his teeth with it. Allah's Messenger (ﷺ) looked at it, so I took the Siwak, cut it (chewed it with my teeth), shook it and made it soft (with water), and then gave it to the Prophet (ﷺ) who cleaned his teeth with it. I had never seen Allah's Messenger (ﷺ) cleaning his teeth in a better way. After finishing the brushing of his teeth, he lifted his hand or his finger and said thrice, "O Allah! Let me be with the highest companions," and then died. `Aisha used to say, "He died while his head was resting between my chest and chin
Reference: Bukhari 4438