Musnad Ahmad 788:269

Yazid narrated to us, he said: Al-Jurayri informed us, from Abu Uthman al-Nahdi, from Abu Musa al-Ash'ari, who said: We were with the Messenger of Allah, peace and blessings of Allah be upon him, on a military expedition. We hastened our return and secured the spoils well. When we overlooked the valley, a man among us began to say the takbir—he said: I believe he said—at the top of his voice. The Messenger of Allah, peace and blessings of Allah be upon him, said: "O people," and he began to gesture with his hand like this—and Yazid described it as if he were gesturing. The Messenger of Allah, peace and blessings of Allah be upon him, said: "O people, you are not calling upon one who is deaf, nor one who is absent. Indeed, the One whom you are calling upon is closer than the heads of your riding beasts." Then he said: "O Abdullah ibn Qais," or "O Abu Musa, shall I not guide you to a word from the treasures of Paradise?" I said: "Yes, O Messenger of Allah." He said: "Say: La hawla wa la quwwata illa billah (There is no power and no strength except by Allah)."