Sunan Ibn Majah, Chapter: The Chapters on Ascetism (Zuhd)

attributing it to the Prophet (saw): "The stronger believer is better and more beloved to Allah than the weak believer, although both are good. Strive to seek that which will benefit you and do not feel helpless. If something overwhelms you, then say: Qaddarallah, wa ma shaa faal (It is the decree of Allah and what He wills He does). And beware of (saying) If only, for If only opens the door to Satan."