Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Strive for that which will benefit you, seek Allah's help and do not lose heart or determination. If anything befalls you, do not say: ‘If only I had done such-and-such’; rather, say: 'Allah has decreed and whatever He wills, He does.’ Saying 'if' opens (the door to) the deeds of the devil.”
شرح الحديث :
Because the religion of Islam calls to work and construction, and to the reform of societies, the Messenger of Allah (may Allah's peace and blessings be upon him) ordered every Muslim to work earnestly, seeking the aid of Allah to actualize the results, and avoiding whatever may hinder him. Meanwhile, he should not open the door to regret or blame if he failed to realize what he worked for, because this will lead him to anger and discontent (with his fate). Rather, one should place his trust in Allah, and believe that whatever happens, whether good or bad, is preordained by Allah, in order to block the way before the devil who would otherwise weaken his faith in Allah, the Almighty, and in His decree and fate.