Musnad Abu Ya'la al-Mawsili 243:32

Narrated to us Khalaf ibn Hisham al-Bazzar, narrated to us Hammad ibn Zayd, from Ayyub, from Abu Uthman, from Abu Musa, who said: We were with the Prophet, may Allah bless him and grant him peace, on a journey. When the people would ascend a high place, they would say "Allahu Akbar" (God is the Greatest). The Prophet, may Allah bless him and grant him peace, said: "O people, be gentle with yourselves, for you are not calling upon one who is deaf or absent, but you are calling upon One who is All-Hearing, Near." He said: He came upon me while I was saying, "La hawla wa la quwwata illa billah" (There is no power and no strength except by Allah). He said: "Indeed, O servant of Allah, shall I not guide you to a treasure from the treasures of Paradise? [It is] 'La hawla wa la quwwata illa billah'."