He related it to the Prophet ﷺ who said, "When one of you supplicates, let him not say, 'O Allah, forgive me if You wish,' but let him be firm in his request, for nothing can compel Him."
حدثنا سفيان عن أبي الزناد عن الأعرج
يبلغ به النبي ﷺ إذا دعا أحدكم فلا يقل اللهم اغفر لي إن شئت ولكن ليعزم بالمسألة فإنه لا مكره له
Its isnad is sahih. Malik related it in Muwatta 213, from Abu Zinad, from Araj, with the wording, "Let none of you say when he prays, 'O Allah, forgive me if You wish, O Allah, have mercy on me if You wish,' rather let him be resolute in his request, for Allah cannot be compelled." Bukhari related it, 11:118, by way of Malik. Muslim related it with similar wording, 2:307, through two other routes from Abu Hurairah.As for "let him be resolute in his request," Ibn Athir said, this means let him strive earnestly in it and be decisive about it.