Musnad Ahmad 1153:2328

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, who said: Ya'qub ibn 'Utbah narrated to me, from al-Zuhri, from 'Urwah, from 'Aishah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, returned on that day when he entered from the mosque, and he lay down in my lap. A man from the family of Abu Bakr entered upon me, and in his hand was a green siwak. She said: The Messenger of Allah, peace and blessings of Allah be upon him, looked at it in his hand with a look by which I knew that he wanted it. She said: I said, "O Messenger of Allah, would you like for me to give you this siwak?" He said, "Yes." She said: So I took it and chewed it for him until I softened it, and I gave it to him. She said: He used it to brush his teeth as intensely as I had ever seen him brush his teeth with a siwak, then he moistened it, then he put it down. And I found the Messenger of Allah, peace and blessings of Allah be upon him, becoming heavy in my lap. She said: I went to look at his face, and behold, his gaze had fixed, and he was saying, "Nay, the Highest Companion from Paradise." I said, "You were given a choice, and you chose, by the One who sent you with the truth." She said: And the Messenger of Allah, peace and blessings of Allah be upon him, passed away.