Musannaf Ibn Abi Shayba 1:1803

Waki’ narrated to us, he said: ‘Abd al-Rahman ibn ‘Amr al-Awza’i narrated to us, from Hassan ibn ‘Atiyyah, who said: Wudu is half of faith, and the siwak is half of wudu. And were it not that it would be difficult for my nation, I would have commanded them to use the siwak at every prayer. Two rak'ahs in which the servant uses the siwak are better than seventy rak'ahs in which he does not use it.