Musannaf Ibn Abi Shayba 21:340

Muhammad ibn Bishr narrated to us, Mis'ar narrated to us, from 'Abd al-Malik ibn Maysarah, from Hilal ibn Yasaf, from 'Amr ibn Maymun, from al-Rabi' ibn Khuthaym, from 'Abdullah, who said: Whoever says ten times: "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 like the equivalent of freeing four slaves—he [the narrator] said: I think he said: "from the children of Ishmael."