ENOfficial
Anas reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "When one of you makes a supplication, he should be firm in the supplication and not say, 'O Allah, if you like, give to me.' Allah cannot be forced against His will."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُحَمَّدُ بْنُ سَلاَمٍ، قَالَ: أَخْبَرَنَا إِسْمَاعِيلُ ابْنُ عُلَيَّةَ، عَنْ عَبْدِ الْعَزِيزِ بْنِ صُهَيْبٍ، عَنْ أَنَسٍ قَالَ: قَالَ رَسُولُ اللهِ صلى الله عليه وسلم: إِذَا دَعَا أَحَدُكُمْ فَلْيَعْزِمْ فِي الدُّعَاءِ، وَلاَ يَقُلِ: اللَّهُمَّ إِنْ شِئْتَ فَأَعْطِنِي، فَإِنَّ اللَّهَ لاَ مُسْتَكْرِهَ لَهُ.
sunnah.com