Musannaf Ibn Abi Shayba 33:842

Yazid ibn Harun narrated to us, from Dawud, from al-Sha'bi, from Abd al-Rahman ibn Abi Layla, from Abu Ayyub, from the Messenger of Allah (may Allah bless him and grant him peace), who said: "Whoever says: 'There is no god but Allah, alone, He has no partner, to Him belongs the dominion, and to Him belongs the praise, in His hand is all good, and He is capable of all things' ten times, it shall be for him equivalent to freeing ten slaves, or equivalent to freeing a slave."