The Prophet (peace and blessings of Allah be upon him) said: “No one of you should say, ‘O Allah, forgive me if You wish, O Allah, have mercy on me if You wish’; he should, rather, be firm in his asking, for Allah cannot be compelled.” (al-Bukhari and Muslim.)
Explaining the reason for such injunction scholars stated that saying inshaa’ Allah denotes the suspension of the request involved in the du`aa’. This may mean that the person is not firm in his or her request. On the other hand, suspending the request in this way may indicate that the person fears that he or she may compel Allah by his or her request, that is, to command Him through the imperative mood used in making du`aa’.
Therefore, one should refrain from saying inshaa’ Allah in du`aa’.