Musannaf Ibn Abi Shayba 21:362

Hussain bin Ali narrated to us, from Za'idah, from Zirr, from Abdullah, who said: Whoever says in his day: "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," it is equivalent for him to freeing four slaves from the children of Ishmael.