Musannaf Ibn Abi Shayba 33:1038

Al-Fadl ibn Dukayn narrated to us, from Abdullah ibn Amir al-Aslami, from Abu al-Zinad, from Said ibn Sulayman, from Zayd ibn Thabit, that the Messenger of Allah (may Allah bless him and grant him peace) used to say: "Shall I not guide you to a treasure from the treasures of Paradise? You should frequently say: 'There is no power and no strength except by Allah' (La hawla wa la quwwata illa billah)."