Musannaf Ibn Abi Shayba 21:335

Ibn Fudayl narrated to us, from Layth, from Talhah, from Abd al-Rahman ibn Awsajah, from al-Bara’ ibn Azib, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever says: 'There is no god but Allah, alone, with no partner, to Him belongs the dominion, and to Him belongs the praise, and He is capable of all things,' it is as the freeing of a slave."