Sunan Ibn Majah, Chapter: The Book of the Sunnah

"The Messenger of Allah (saw) said: "The strong believer is better and more beloved to Allah than the weak believer, although both are good. Strive for that which will benefit you, seek the help of Allah, and do not feel helpless. If anything befalls you, do not say, "If only I had done such and such." Rather say, "Qaddara Allahu wa ma sha'a fa'ala (Allah has decreed and whatever He wills, He does)." For (saying) 'If' opens (the door) to the deeds of Satan.'" (Sahih)