Musnad Abu Ya'la al-Mawsili 150:234

Abu Hisham al-Rifa’i narrated to us, Ishaq narrated to us, Mu’awiyah narrated to us, from al-Zuhri, from ‘Urwah, from ‘Aishah, who said: The Messenger of Allah (ﷺ) used to prefer the prayer for which he used the siwak over the prayer for which he did not use the siwak by seventy degrees. And the Messenger of Allah (ﷺ) used to prefer the hidden dhikr, which the recording angels do not hear, by seventy degrees. He said: When the Day of Resurrection comes and Allah gathers the creatures for their reckoning, and the recording angels come with what they have memorized and written, Allah will say to them: "Look, does he have anything remaining?" They will say: "Our Lord, we have not left anything that we knew and memorized except that we have counted it and written it down." Then Allah, Blessed and Exalted is He, will say to him: "Indeed, you have with Me a hidden [deed] that you do not know, and I will reward you for it, and it is the hidden dhikr."