It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that a man of the Ansar from the tribe of Banu Amr bin Awf said: "O Messenger of Allah, you have greatly encouraged us to use the siwak; is there anything else in its place?" He (peace and blessings be upon him) said: "Your fingers are a siwak at the time of your ablution; rub them over your teeth. Indeed, there is no deed (accepted) for one who has no intention, and there is no reward for one who does not hope for reward from Allah Almighty."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 179