Sahih Bukhari Volume 8, Book 80, Hadith 6340

حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا إِسْمَاعِيلُ، أَخْبَرَنَا عَبْدُ الْعَزِيزِ، عَنْ أَنَسٍ ـ رضى الله عنه ـ قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ إِذَا دَعَا أَحَدُكُمْ فَلْيَعْزِمِ الْمَسْأَلَةَ، وَلاَ يَقُولَنَّ اللَّهُمَّ إِنْ شِئْتَ فَأَعْطِنِي‏.‏ فَإِنَّهُ لاَ مُسْتَكْرِهَ لَهُ ‏"‏‏.‏

Narrated By Anas : Allah's Apostle said, "When anyone of you appeal to Allah for something, he should ask with determination and should not say, 'O Allah, if You wish, give me.', for nobody can force Allah to do something against His Will.