Majma al-Zawaid 2574 — Using the siwak for one who has no teeth

Sayyidah Aishah Siddiqah (may Allah be pleased with her) narrates: I said: "O Messenger of Allah, a man whose mouth has gone (that is, whose teeth are gone), should he use the miswak?" The Prophet (peace and blessings be upon him) said: "Yes." I asked: "How should he do it?" He said: "He should put his finger into his mouth and rub it."
Al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains a narrator, Isa bin Abdullah al-Ansari, who is da'if (weak).