Sayyiduna Anas (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "I was commanded to use the miswak until I feared that I would lose my teeth (the narrator is unsure; perhaps the words are:) until I feared for my gums and my teeth."
Al-Bazzar recorded it; its chain contains a narrator, Imran bin Khalid, who is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2561