Musnad Abu Ya'la al-Mawsili 130:54

Ishaq narrated to us, Hammad narrated to us, from Abd al-Aziz, from Anas, that the Messenger of Allah (may Allah bless him and grant him peace) said: "O Mu'adh." He said: "At your service, O Messenger of Allah." Then he said: "O Mu'adh." He said: "At your service, O Messenger of Allah." He said it three times, and each time he said: "At your service, O Messenger of Allah." He said: "Give glad tidings to the people that whoever says: 'There is no god but Allah,' will enter Paradise."