From one of the Companions of the Prophet (peace and blessings be upon him), this saying of the Prophet (peace and blessings be upon him) is reported: "Were it not that I feared putting my Ummah to hardship, I would have ordered them to use the tooth-stick (siwak) with every prayer."
Ahmad recorded it, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2546