Musnad Ahmad 27:1413

Qutaybah ibn Sa'id narrated to us, Ibn Lahi'ah narrated to us, from 'Ubayd Allah ibn Abi Ja'far, from Nafi', from Ibn 'Umar, that the Prophet, peace and blessings of Allah be upon him, said: "You must use the siwak (tooth stick), for it is a purification for the mouth and a means of seeking the pleasure of the Lord."