Musnad Ahmad 456:20

'Abd al-Samad narrated to us, he said: Harb—meaning Ibn Shaddad—narrated to us, from Yahya, Abu Salamah narrated to us. And Muhammad ibn Fudayl narrated to us, from Muhammad ibn Ishaq, from Muhammad ibn Ibrahim, from Abu Salamah, from Zayd ibn Khalid al-Juhani, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Were it not that I would burden my nation, I would have commanded them to use the siwak (tooth-stick) at every prayer." He said: Zayd ibn Khalid used to place his siwak in his ear just as a scribe places a pen behind his ear; whenever he stood for prayer, he would use the siwak.