Musnad Ahmad 30:1670

Khalaf ibn al-Walid narrated to us, he said: 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, or more excellent, and more beloved to Allah than the weak believer, and in each there is good. Be eager for that which benefits you, and do not be incapable. If something overcomes you, then say: 'The decree of Allah, and whatever He wills, He does.' And beware of 'if only,' for 'if only' opens [the way] for the devil."