Musannaf Ibn Abi Shayba 21:334

Yazid ibn Harun narrated to us, from Dawud, from al-Sha'bi, from Abd al-Rahman ibn Abi Layla, from Abu Ayyub al-Ansari, 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 the freeing of ten slaves, or a slave."