Musnad Ahmad 1060:49

Yazid narrated to us, Dawud informed us, from 'Amir, from 'Abd al-Rahman ibn Abi Layla, from Abu Ayyub, that the Messenger of Allah (may Allah bless him and grant him peace) 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, and He is capable of all things,' ten times, it shall be for him equivalent to the freeing of ten slaves," or "a slave."