ENOfficial
He reported God’s messenger as saying, “When one of you makes supplication he must not say, ‘O God, forgive me if Thou wilt, but must be definite and cherish a great desire, for nothing which God gives is burdensome to Him.” Muslim transmitted it.
Source: sunnah.com
Other translations
Arabic
وَعَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: " إِذَا دَعَا أَحَدُكُمْ فَلَا يَقُلِ: اللَّهُمَّ اغْفِرْ لِي إِنْ شِئْتَ وَلَكِنْ لِيَعْزِمْ وَلْيُعَظِّمِ الرَّغْبَةَ فَإِنَّ اللَّهَ لَا يَتَعَاظَمُهُ شيءٌ أعطاهُ ". رَوَاهُ مُسلم
sunnah.com