It was narrated that Abu Hurairah said: "The Messenger of Allah (p) said:
الْمُؤْمِنُ الْقَوِيُّ خَيْرٌ وَأَحَبُّ إِلَى اللَّهِ مِنَ الْمُؤْمِنِ الضَّعِيفِ وَفِي كُلٍّ خَيْرٌ احْرِصْ عَلَى مَا يَنْفَعُكَ وَاسْتَعِنْ بِاللَّهِ وَلاَ تَعْجِزْ فَإِنْ أَصَابَكَ شَىْءٌ فَلاَ تَقُلْ لَوْ أَنِّي فَعَلْتُ كَذَا وَكَذَا . وَلَكِنْ قُلْ قَدَّرَ اللَّهُ وَمَا شَاءَ فَعَلَ فَإِنَّ " لَوْ " تَفْتَحُ عَمَلَ الشَّيْطَانِ " .
'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.'" (Sunan Ibn Majah, The Book of the Sunnah, Sahih)