Musnad Ahmad 1153:1622

Ibrahim ibn Khalid narrated to us, he said: Rabah narrated to us, from Ma'mar, from Hisham ibn 'Urwah, from his father, from 'Aishah, that she said: The Prophet, peace and blessings of Allah be upon him, was leaning his back against me when he was taken [in death]. She said: Then 'Abd al-Rahman ibn Abi Bakr entered, and in his hand was a siwak. The Prophet, peace and blessings of Allah be upon him, asked for it, so I took the siwak and softened it, then I handed it to him. He began to use it to brush his teeth, then his hand became heavy and he became heavy upon me. He was saying: "O Allah, in the Highest Companion. O Allah, in the Highest Companion," twice. She said: Then he was taken [in death]. 'Aishah says: "The Messenger of Allah, peace and blessings of Allah be upon him, was taken [in death] while he was between my chest and my throat."