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Al-Adab al-Mufrad 607
حَدَّثَنَا مُحَمَّدُ بْنُ عُبَيْدِ اللهِ، قَالَ: حَدَّثَنَا عَبْدُ الْعَزِيزِ بْنُ أَبِي حَازِمٍ، عَنِ الْعَلاَءِ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللهِ صلى الله عليه وسلم قَالَ: إِذَا دَعَا أَحَدُكُمْ فَلاَ يَقُولُ: إِنْ شِئْتَ، وَلْيَعْزِمِ الْمَسْأَلَةَ، وَلْيُعَظِّمِ الرَّغْبَةَ، فَإِنَّ اللَّهَ لاَ يَعْظُمُ عَلَيْهِ شَيْءٌ أَعْطَاهُ.
Abu Hurayra reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "When one of you makes a supplication, he should not say, 'If you wish.' He should be firm in asking and he should have great hope. Allah does not think that anything that He gives is too great.'"
Cited in Al-Adab al-Mufrad (No. 607).