Musnad Ahmad 788:251

Yunus ibn Muhammad narrated to us, he said: Hammad ibn Zayd narrated to us, Ghaylan ibn Jarir narrated to us, from Abu Burdah, from Abu Musa, who said: "I entered upon the Messenger of Allah (peace and blessings of Allah be upon him) while he was using the siwak, and he was placing the tip of the siwak upon his tongue, brushing upwards." Hammad described it: As if he were raising his siwak. Hammad said: And Ghaylan described it to us, saying: He used to brush lengthwise.