Musnad Abu Ya'la al-Mawsili 219:22

Narrated to us Rawh ibn Abd al-Mu'min al-Muqri, narrated to us Ya'qub ibn Ibrahim ibn Sa'd ibn Ibrahim al-'Awfi, narrated to us my father, from Muhammad ibn Ishaq, from Muhammad ibn Talhah ibn Yazid ibn Rukanah, from Salim ibn Abd Allah, from Abu al-Jarrah, the freed slave of Umm Habibah, the wife of the Prophet (may Allah bless him and grant him peace), that Umm Habibah bint Abi Sufyan narrated to him: That the Messenger of Allah (may Allah bless him and grant him peace) said: "Were it not that I would make it difficult for my nation, I would have commanded them to use the siwak (tooth stick) at every prayer, just as they perform ablution."