Musnad Abu Ya'la al-Mawsili 140:29

Abu Hisham al-Rifa'i narrated to us, al-Muharibi narrated to us, from Layth, from Yazid al-Raqashi, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever seeks refuge in Allah ten times a day from the Shaytan, Allah will appoint an angel over him to repel the devils from him."