Ash Shaykh Abu Amr Al Hajooree حفظه الله mentioned,
“It has become widespread among the people that a man supplicates for his brother and says, ‘May Allah reward you Inshaa Allah’, or, ‘May Allah bless you Inshaa Allah’, or, ‘May Allah make it easy for you Inshaa Allah’, or, ‘May Allah cure you Inshaa Allah’, or similar to these [supplications], and this is Haram (prohibited). This is Haram and not permissible that you say Inshaa Allah. Rather, you say ‘May Allah reward you’, ‘May Allah cure you’, ‘May Allah give you’. This is due to that which comes in
the two Saheehs (Al Bukhari and Muslim), from the narration of Abu Huraira and Anas, may Allah be pleased with both of them, that the prophet صلى الله عليه و سلم said, ‘None of you should say, “O Allah! Forgive me if You wish,” or, “Bestow Your Mercy on me if You wish,” or, ‘Provide me with means of subsistence if You wish”, but rather he should be firm in his request, for Allah does what He wills and nobody can force Him (to do anything)’
Thus [regarding] your statement, ‘Oh Allah give me if you want’, then Allah Aza Wa Jal is never forced. He isn’t incapable, nor weak and you are not able to be self sufficient from Allah. Thus be firm in your request. Make constant supplications, supplicate with good, supplicate to Allah that he give you Firdaus Al A’alaa (the highest part of paradise). Supplicate to Allah that he repel evil from you, supplicate to Allah for good in this world and the hereafter, and be firm in your request. This action of saying Inshaa Allah (if Allah wills) in supplications is a mistake, rather it is prohibited.
As for saying Inshaa Allah in regards to future matters, then it is obligatory. You say for example, ‘tomorrow I will go to Khartoum inshaa Allah’. It is obligatory that you say Inshaa Allah in this situation, due to the statement of Allah Aza Wa Jaal
وَلَا تَقُولَنَّ لِشَيْءٍ إِنِّي فَاعِلٌ ذَلِكَ غَداً . إِلَّا أَن يَشَاءَ اللَّهُ
And never say of anything, ‘I shall do such and such thing tomorrow.’ Except (with the saying), ‘If Allah will!’
Thus if it is in regards to matters of the future, whether that be tomorrow or today, for example you saying ‘Oh so and so, tonight I will come to you around Asr’, then say Inshaa Allah. As an obligation, you say Inshaa Allah in regards to that which will occur in the future.
And (to say Inshaa Allah) in regards to matters which have passed, then it is an innovation. You saying ‘yesterday I went to the marketplace Inshaa Allah’ is an innovation to say Inshaa Allah [in this situation]. Rather, you should say, ‘I went’ because you went and it has already occurred. As for matters of the future, then it is obligatory (to say Inshaa Allah).
How many matters are these? Three.
In supplications, saying Inshaa Allah is prohibited. Rather you should supplicate, and if your supplication has transgression in it, then don’t make that supplication. As for if it is a legislated supplication, then say, ‘Oh Allah give me, Oh Allah cure me’ and don’t say Inshaa Allah.
In matters connected to the future, it is obligatory to say ‘Inshaa Allah, I will do such and such Inshaa Allah’.
And in matters connected to the past, then it is an innovation, it is an innovation to say, ‘I did such and such Inshaa Allah.’ ” End translation.
Majid Jawed Al Afghanee