Praise be to Allah.
Du‘aa’s are of two types:
The first type is du‘aa’ that is specific to a particular time, place or act of worship, is to be recited a specific number of times, or has a particular virtue, such as the du‘aa’ recited at the beginning of the prayer, the du‘aa’ for entering the washroom, du‘aa’s that are recited when going to sleep, the du‘aa’ for entering the mosque, and so on.
With regard to this type, it is not permissible to introduce any du‘aa’ other than that which is narrated in the religious texts. Just as it is stipulated to be sincere towards Allah, may He be glorified and exalted, in order for it to be acceptable, it is also stipulated that it be done following the example of the Prophet (blessings and peace of Allah be upon him).
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
With regard to adopting a wird [daily litany of dhikr and du‘aa’] that is not prescribed in the Islamic texts, or regularly reciting a dhikr that is not prescribed, this is something that is not allowed. Moreover, the du‘aa’s and adhkaar that are prescribed are all that one needs to attain sound goals and sublime objectives, and no one ignores them in favour of other invented, innovated adhkaar except one who is ignorant or heedless or overstepping the bounds. End quote.
Majmoo‘ al-Fataawa (22/511).
Al-‘Allaamah al-Mu‘allimi (may Allah have mercy on him) said:
What a great loss is incurred by the one who turns away from the du‘aa’s that are proven in the Book of Allah, may He be glorified and exalted, or in the Sunnah of the Messenger of Allah (blessings and peace of Allah be upon him), so he hardly recites them in du‘aa’, then he prefers du‘aa’s and recites them regularly. Is this not wrongdoing and overstepping the mark? End quote.
What you should do is adhere to that which was narrated from the Prophet (blessings and peace of Allah be upon him) in saheeh (sound) hadiths of du‘aa’s that can be recited at various times and in various circumstances.
Hence the scholars took care to compile the du‘aa’s that were narrated from the Prophet (blessings and peace of Allah be upon him) so that they might be readily available to the people, and thus they would have no need for innovated du‘aa’s that are not proven.
Imam at-Tabaraani (may Allah have mercy on him) said in the introduction to his book ad-Du‘aa’ (22):
I wrote this book to bring together the du‘aa’s of the Messenger of Allah (blessings and peace of Allah be upon him). What prompted me to do that was the fact that I saw many people consistently reciting du‘aa’s in rhymed prose, or du‘aa’s that were fabricated by some storytellers, who allocated a different du‘aa’ for different days, which were not narrated from the Messenger of Allah (blessings and peace of Allah be upon him) or from any of his Companions, or from any of those who followed him in truth, despite the reports narrated from the Messenger of Allah (blessings and peace of Allah be upon him) stating that it is disliked to compose du‘aa’s in the form of rhymed prose and go to extremes in doing so. So I wrote this book, with isnaads that go back to the Messenger of Allah (blessings and peace of Allah be upon him. End quote.
Please see the answer to question no. 11017.
The second type is du‘aa’ in general terms, which refers to du‘aa’s that a person may say in situations and at times concerning which no specific du‘aa’ is mentioned in Islamic teachings, such as du‘aa’ in the last third of the night, and so on.
In this case, there is no specific du‘aa’ that is prescribed in Islam; rather it is left to the choice of each person who is offering du‘aa’ to ask Allah, may He be exalted, for what he needs. With regard to this type, there is nothing wrong with learning from the du‘aa’s of the righteous, or the format of du‘aa’s mentioned in some da‘eef hadiths, because they may contain concise and comprehensive words, good words of praise of Allah, and good ways of asking that are easy for the Muslim to say. That is a condition that there be nothing objectionable in these words, and that one does not believe that there is any special virtue in them, or adhere to reciting them in du‘aa’ all the time. But if a person recites them sometimes, there is nothing wrong with that; however reciting them all the time means that one is treating that as if it is of the same status as a prescribed du‘aa’.