Musannaf Ibn Abi Shayba 21:693

Ya'la ibn 'Ubayd narrated to us, from Hajjaj ibn Dinar, from al-Hakam ibn Jahal, from a man who narrated to him from 'Ali, that he said: Whoever recites "Say, He is Allah, [who is] One" ten times after the dawn [prayer], no sin will reach him that day, even if the devils strive against him.