Musnad Abu Ya'la al-Mawsili 147:58

Musa ibn Muhammad ibn Hayyan narrated to us, Abd al-Malik ibn Amr narrated to us, Sa'id ibn Sulaym al-Dabbi narrated to us, he said: I heard Anas ibn Malik saying: When the Messenger of Allah (may Allah bless him and grant him peace) would travel or go on a military expedition, he would have one of his companions ride behind him every day. He said: So the one riding behind the Messenger of Allah (may Allah bless him and grant him peace) was Mu'adh ibn Jabal. He called out to him while he was riding behind him and said: "O Mu'adh ibn Jabal." I said: "At your service, O Messenger of Allah." He said: "Do you know what is the right of Allah upon the servants? That they testify that there is no god but Allah and that Muhammad is His servant and His messenger, and that they worship Him and do not associate anything with Him." He repeated this hadith three times. Then he called out and said: "O Mu'adh." He said: "At your service, O Messenger of Allah." He said: "Do you know what is the right of the servants upon Allah if they do that?" He said: "Allah and His Messenger know best." He said: "Indeed, the right of the servants upon Allah, if they do that, is that He does not punish them and that He enters them into Paradise."