Imam Ahmad recorded this report from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both); he narrates that the Prophet (peace and blessings be upon him) said: "I was commanded to use the miswak until I feared that revelation would be sent to me about it."
Its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2557