Musnad Ahmad 482:150

Hassan narrated to us, Ibn Lahi'ah narrated to us, Abu 'Ushaynah narrated to us, from 'Uqbah ibn 'Amir, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Your Lord marvels at a shepherd of sheep on the peak of a mountain crag who calls the Adhan for prayer and prays. So Allah says: 'Look at this servant of Mine; he calls the Adhan and establishes the prayer, fearing Me. I have forgiven him and admitted him into Paradise.'"