Narrated Anas bin Malik radiAllahi anhu, Rasulullah Salla Allahu Alaihi wa sallam 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.

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 ‏”

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Sahih al-Bukhari 6338
In-book : Book 80, Hadith 35
USC-MSA web (English) : Vol. 8, Book 75, Hadith 350 (deprecated)