Hadith 706
It is narrated from Sayyiduna Abu Hurairah, may Allah be pleased with him, that the Prophet, peace and blessings be upon him, said: "If it were not difficult for my Ummah, I would have commanded them to use (the miswak) at every prayer time."
Imam Darimi Abu Muhammad said: The reference of «لأَمَرْتُهُمْ بِهِ» is to the miswak. (In the Sahihain, instead of the pronoun, the word 'siwak' itself is mentioned. — Translator)