Reference: Ahmad 4/403. See also Al-Albani, Sahihul-Jami’ As-Saghir 3/233 and Sahihut-Targhib wat- Tarhib 1/19.
Posted from: Explaining Shirk: The Greatest Sin – Muhammad Ibn Sālih al-’Uthaymīn, Sālih al-Fawzān
Read for the Source and Explanation of the above du’a @ Du’aa for Fear of Shirk