Sayyiduna Uqbah bin Amir al-Juhani (may Allah be pleased with him) said: I was with the Messenger of Allah (peace and blessings be upon him) in an army. I let my companions' animals out to graze, and when I returned, my companions came hurrying and saying: While we were with the Messenger of Allah (peace and blessings be upon him), the mu'adhdhin called the adhan, and he (peace and blessings be upon him) said: "I bear witness that there is no god but Allah, and I bear witness that Muhammad is the Messenger of Allah." Then the narrator mentioned a hadith similar to that.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15019