Riyad Us-Saliheen (Gardens of the Righteous)

Chapter 332
Abomination of saying: "Forgive me if you wish, O Allah!"

1743. Abu Hurairah (May Allah be pleased with him) said: The Messenger of Allah (PBUH) said, "You must not supplicate: `O Allah! forgive me if You wish; O Allah bestow mercy on me if You wish.' But beg from Allah with certitude for no one has the power to compel Allah.''
[Al-Bukhari and Muslim]

Another narration of Muslim is: "A supplication should be made in full confidence and one should persistently express his desire (before Allah) in his supplication, for no bounty is too great for Allah to bestow (upon his slaves).''

Commentary:  "Beg from Allah with certitude'' means that one should not pray with `ifs' and `buts' but should make a definite prayer and insist upon it. "For no bounty is too great for Allah'' means that Allah has absolute power to answer our prayers, whether they relate to religious or worldly matters.

1744. Anas (May Allah be pleased with him) said: The Messenger of Allah (PBUH) said, "When one of you supplicates, let him be decisive and he should not say: `O Allah, bestow upon me such and such if You wish', because no one has the power to compel Him.''
[Al-Bukhari and Muslim].

Commentary: One should pray to Allah will full confidence that He will answer his prayers. One should also persist in praying and never give in to despair.