Sayyidah Zaynab bint Jahsh (may Allah be pleased with her) narrates: I heard the Prophet (peace and blessings be upon him) say: "Were it not that I feared putting my Ummah to hardship, I would have ordered them to use the tooth-stick (siwak) at every prayer, just as they perform ablution (at every prayer)."
Ahmad recorded it, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2545