Musnad Ahmad 1274:3

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, Muhammad ibn Talhah ibn Yazid ibn Rukanah narrated to me, from Salim ibn Abdullah ibn Umar, from Abu al-Jarrah, the freed slave of Umm Habibah, the wife of the Prophet, may Allah bless him and grant him peace, from Umm Habibah, that she narrated to him from Zaynab bint Jahsh, she said: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "Were it not that it would be difficult for my nation, I would have commanded them to use the siwak at every prayer, just as they perform ablution."