Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And with this chain of narration, it is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that he said: If the Messenger of Allah (peace and blessings be upon him) had not considered the hardship of his ummah, he would have commanded (the use of) the miswak with every prayer or every ablution. (The narrator and summarizer of this book, Imam) Abu al-Hasan (al-Qabisi, may Allah have mercy on him) said: There is a problem regarding the attribution of this wording (text) as marfu‘ to the Prophet (peace and blessings be upon him), but ‘Isa ibn Miskin said (in his narration) that the Messenger of Allah (peace and blessings be upon him) said: “If I had not feared hardship for my ummah, I would have commanded them (to use the miswak).”

And with the same chain of narration, it is narrated from Sayyiduna Abu Hurairah, may Allah be pleased with him, that the Messenger of Allah, peace and blessings be upon him, said: "If I did not fear hardship for the people or for the believers, I would have certainly commanded them to use the tooth-stick (miswak)."