“…and what is commonly narrated from Ahmad is that one should not supplicate in the prayer except with those supplications that are specifically legislated and narrated from the Prophet, as al-Athram said:
“I asked Ahmad: “With what should I supplicate after the tashahhud?” So, Ahmad said: “With what is narrated.” So, I said to him: “Did the Messenger of Allah not say: “…then let him choose from the supplications whatever he wishes…”?” Ahmad said: “He chooses from the narrated supplications whatever he wishes,”” – and this is the meaning of the words of Ahmad.
And I have previously clarified some of the principles regarding this, because of His Saying: {“Verily, Allah does not love the transgressors…”} [al-A’raf; 55], and because not all forms supplication are allowed. Rather, from the supplications are those that constitute excessiveness and transgression, and are forbidden, and the supplications which are legislated contain no transgression. Transgression in supplication can sometimes be in the form of excessive wording, and can sometimes be in regards to the meaning, just as one of the Companions explained to his son when some said: ‘O Allah! I ask You for a white palace in the right section of Paradise if I enter it,’ and another said: ‘I ask You for the Paradise, its palaces, and its rivers, and I seek refuge with You from the Fire, its chains, and its shackles.’ So, the man said to his son: “My son! Just ask Allah for Paradise, and seek refuge with Him from the Fire, as I heard the Messenger of Allah say: “There will be in this nation those who are excessive in supplication and purification.”“
And transgression can occur in regards to worship and zuhd, and the statement of Ahmad regarding limiting oneself to the narrated supplications is a good one, as the point is to make a supplication that is loved by Allah, not just supplicating for the sake of supplicating, as from the supplications are those that are forbidden.
So, if it is said that the supplications that are allowed outside of prayer are also allowed during the prayer – such as one asking for a house, or a beautiful slave girl – it is then asked: who even said that such types of supplications are legislated outside of the prayer, and that such wordings do not constitute transgression and excessiveness? Here, it should be said that the supplication that is mustahabb is that which is legislated, as istihbab of a particular act is determined by the Legislator. So, whatever He has not specifically legislated is not mustahabb. Rather, it constitutes legislating in the Religion that which Allah did not allow, and supplication is the greatest aspect of this religion.
However, if one supplicates with something that he does not know is mustahabb, or knows that it is allowed without being mustahabb, his prayer is not invalidated due to this, as prayer is invalidated due to the speech of the people, and supplication is not from the speech of the people. Rather, it is just as if one were to praise Allah in a wording that was not specifically legislated, and this was reported from some of the Companions during the time of the Prophet, and he did not reprimand them for praising Allah in a manner that was not legislated at that time. Rather, one would only be deprived of the reward if they were to supplicate like this.
And from the supplication is that which is makruh without invalidating the prayer, as well as that which would invalidate the prayer, and supplications can be divided into five types:
– those that are legislated, and these include the wajib and mustahabb;
– those that are mubah (allowed) and are not mustahabb, and these do not invalidate the prayer;
– those that are makruh, and these do not invalidate the prayer, and are like if one were to turn his face in the prayer, say the tashahhud during the standing, or recite the Qur’an during the sitting;
– those that are forbidden, and these invalidate the prayer, as they are considered to be human speech;
So, this is an analysis of the statement of Ahmad, as the prayer is not invalidated by making a supplication that is not narrated from the Prophet. However, it is not mustahabb to do so, as nothing can be mustahabb that is not legislated. He also clarified that the choice given is from those supplications that are legislated, and that the legislated supplications can be in the form of the exact wording, or they can be in the form of the general meaning, provided that the Prophet did not restrict the supplication to just one wording, as in the case of the Qur’an.”
[‘Majmu’ al-Fatawa’; 22/231-232]
Shaykh-ul-Islaam Ibn Taymiyyah 