Al-Nadr bin Anas narrates from his father (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The fingers suffice in place of the siwak."
Isa (bin Shu'ayb) alone narrated it with both chains.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 177