Musnad Ahmad 1049:1

Yahya ibn Sa'id narrated to us, he said: We heard it from al-A'mash, 'Abdullah ibn Yasar narrated to me, from 'Abd al-Rahman ibn Abi Layla, from a man among the companions of the Prophet, may Allah bless him and grant him peace, from the Prophet, may Allah bless him and grant him peace, who said: "Were it not that I would make it difficult for my nation, I would have commanded them to use the siwak with every prayer."