Musannaf Ibn Abi Shayba 33:837

Muhammad ibn Bishr narrated to us, he said: Mis'ar narrated to us, from 'Abd al-Malik ibn Maysarah, from Hilal ibn Yasaf, from 'Amr ibn Maymun, from Rabi' ibn Khuthaym, from 'Abdullah ibn Mas'ud, who said: Whoever says ten times: "There is no god but Allah, 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 believe he said: "from the children of Ishmael."