Musnad Ahmad 25:1293

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, he said: I heard Abu Ishaq narrating that he heard a man from Banu Tamim say: I asked Ibn Abbas about a man pointing with his finger like this—meaning during prayer—he said: "That is sincerity." And Ibn Abbas said: "The Messenger of Allah (ﷺ) commanded us to use the siwak, until we thought that a revelation would descend upon him regarding it." And: "I saw the Messenger of Allah (ﷺ) prostrating until the whiteness of his armpits could be seen."