It is narrated from al-Tamimi: I asked Ibn Abbas (may Allah be pleased with them both) about the miswak, and he said: "The Prophet (peace and blessings be upon him) kept commanding us to use it, until we feared that it might become obligatory."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 144