(3) باب الْعَزْمِ بِالدُّعَاءِ وَلاَ يَقُلْ إِنْ شِئْتَ
Abu Huraira reported Allah's Messenger (ﷺ) as saying:
None of you should say to Allah (like this): O Allah, grant me mercy, if thou so likest. The supplication (of his) should (be permeated with) conviction (that it would be accepted by the Lord), for Allah is the Doer of (everything) He likes to do, and there is none to force Him (to do or not to do this or that).
Reference: Sahih Muslim 2679 b
In-book reference: Book 48, Hadith 9
USC-MSA web (English) reference(deprecated numbering scheme): Book 35, Hadith 6479