Musannaf Ibn Abi Shayba 21:188

Waki' narrated to us, from Mis'ar, from 'Affan, from 'Amr ibn Maymun, who said: Whoever says when he goes to his bed, "I bear witness that there is no god but Allah alone, He has no partner, to Him belongs the dominion, and to Him belongs the praise, and He is capable of all things," four times, his sins will be forgiven, even if they were the fill of the earth.