مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3128
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
Abu Ishaq narrated that he heard a man of Banu Tameem say: I asked Ibn ‘Abbas (رضي الله عنه) about a man doing this with his fingers - in the prayer - and he said: That is sincerity. And Ibn 'Abbas said: The Messenger of Allah (ﷺ) enjoined us to use the siwak, to such an extent that we thought that revelation would come down to him concerning it. And I saw the Messenger of Allah (ﷺ) prostrating and I saw the whiteness of his armpits.
Musnad Ahmad 3152Hasan because of corroborating evidence, this is a da'eef is