It was narrated that Abu Hurairah said:

"The Messenger of Allah (ﷺ) 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.'"

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَعَلِيُّ بْنُ مُحَمَّدٍ الطَّنَافِسِيُّ، قَالاَ حَدَّثَنَا عَبْدُ اللَّهِ بْنُ إِدْرِيسَ، عَنْ رَبِيعَةَ بْنِ عُثْمَانَ، عَنْ مُحَمَّدِ بْنِ يَحْيَى بْنِ حَبَّانَ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ ‏"‏ 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 ‏"‏ ‏.‏