Musnad Abu Ya'la al-Mawsili 159:98

Khalid ibn Mirdas Abu al-Haytham narrated to us, Abdullah ibn al-Mubarak narrated to us, from Muhammad ibn Ajlan, from Rabi'ah, from al-A'raj, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The strong believer is better, more excellent, and more beloved to Allah than the weak believer, and in each there is good. Strive for that which benefits you and do not be helpless. If a matter overcomes you, then say: 'The decree of Allah, and whatever He wills, He does.' And beware of 'if,' for 'if' opens the work of Satan."