Musnad Ahmad 30:1287

Yahya ibn Yazid ibn Abd al-Malik narrated to us, from his father, from Sa'id ibn Abi Sa'id, from Abu Hurairah, that the Prophet, may Allah bless him and grant him peace, said: "Increase in saying 'La hawla wa la quwwata illa billah' (There is no power and no strength except by Allah), for it is a treasure from the treasures of Paradise."