Sayyiduna al-Abbas (may Allah be pleased with him) narrates: (Some) people came to the Prophet (peace and blessings be upon him) without having used the siwak, so he said: You people come to me just like that; you do not use the siwak. Were it not that I feared hardship for my Ummah, I would have made the siwak obligatory upon them just as I have made ablution obligatory upon them. Sayyidah Aishah (may Allah be pleased with her) says: The Prophet (peace and blessings be upon him) kept mentioning the siwak so much that we feared that Qur'an might be revealed concerning it.
Imam Abu Ya'la, Imam al-Bazzar and Imam al-Tabarani (in al-Mu'jam al-Kabir) recorded this report; its chain contains a narrator, Abu Ali al-Sayqal, and he is majhul (unknown). Allamah al-Haythami says: I say: Many more reports will come, in the chapter on using the siwak and in the chapter on matters related to the prayer, if Allah wills.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1122