It is narrated from al-Nadr bin Anas bin Malik (may Allah have mercy on him) that the Messenger of Allah (peace and blessings be upon him) said: "The fingers suffice in place of the siwak."
This is how I found it in the book of Isa bin Shu'ayb; what is preserved (mahfuz) from the hadith of Ibn al-Muthanna is the narration that follows.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 178