It is narrated from Sayyidah Aishah (may Allah be pleased with her) that she said: Among Allah's favours upon me and His good trial of me is that the Messenger of Allah (peace and blessings be upon him) passed away in my house and on my day, and his (peace and blessings be upon him) soul was taken between my chest and my neck, and on the day he (peace and blessings be upon him) passed away my saliva and his (peace and blessings be upon him) saliva were joined. The narrator said: I asked her: That much concerns your house, your day, and your chest and neck, but how were your saliva and his saliva joined? She said: On Monday morning Abd al-Rahman bin Abi Bakr came in to me with a fresh miswak in his hand. I saw that the gaze of the Messenger of Allah (peace and blessings be upon him) was fixed on it, and the Messenger of Allah (peace and blessings be upon him) was very fond of the miswak. I said: Split it and give it to me. He split it into two and gave it to me, so I chewed it to soften it, then put it into the mouth of the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) cleaned his teeth with it, and in this way my saliva and his (peace and blessings be upon him) saliva were joined.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19384