Praise be to Allah.
If what is meant is the du‘a that is published widely in many chatrooms and on many websites under the name “al-du‘a al-mustajaab”, which is attributed to a report that says that ‘Umar ibn al-Khattab and ‘Ali ibn Abi Talib (may Allah be pleased with them) said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “Whoever offers supplications calling upon Allah by these names, He will answer his supplication: ‘O Allah, You are the Ever Living Who never dies, the Creator Who never is overcome, the All-Seeing Who is never in doubt, the All-Hearing Who is never uncertain, the Truthful One Who never lies…’”… to the end of the lengthy hadeeth (prophetic narration):
This is a fabricated du‘a that is falsely attributed to the Messenger of Allah (peace and blessings Allah be upon him) and there is no saheeh (authentic) hadeeth that speaks of its virtue. It is not permissible for any Muslim to transmit it on the basis of it being a hadeeth from the Messenger of Allah (peace and blessings of Allah be upon him). We have previously discussed this in detail on our website, in the answer to question no. 116137.
If what this sister is referring to is the book which is called ad-Du‘a al-Mustajab, it is also an unreliable book which includes many fabricated hadeeths and a number of things that are contrary to sharee‘ah (Islamic teachings), hence the scholars have warned people against it.
In Fatawa al-Lajnah ad-Da’imah (2/449) it says:
This book – ad-Du‘a al-Mustajab – cannot be relied upon because it contains a large number of weak (da‘eef) and fabricated (mawdoo‘) hadeeths. End quote.
‘Abd al-‘Azeez ibn ‘Abdullah ibn Baz, ‘Abd ar-Razzaq ‘Afeefi, ‘Abdullah ibn Ghadyan, ‘Abdullah ibn Qa‘ood.
Shaykh Ibn Baz (may Allah have mercy on him) said:
It is a book that cannot be relied upon and its author is not a scholar. Hence we have been drawing attention to this for a long time and we have stated that this book cannot be relied upon. Readers should be alerted to the fact that it cannot be relied upon because of what it contains of weak and fabricated hadeeths.
Fatawa Ibn Baz, 26/354-355
Shaykh Salih al-Fawzan (may Allah preserve him) said:
There is a book that is widely circulated among people today and is available in bookstores. It is called ad-Du‘a al-Mustajab, by Muhammad ‘Abd al-Jawad. It is a book of myths which includes innovated Sufi du‘as and dhikrs (words of remembrance) which are quoted without any evidence for them from the Quran and Sunnah. Its title is an audacious statement about Allah, may He be glorified and exalted, because the book is called ad-Du‘a al-Mustajab (the supplication that is answered). But who is the one who told him that this supplication is answered? What evidence is there for this, except that he wanted to attract the attention of the common folk and deceive them with this fabricated book. I refer the reader to some trustworthy books of du‘aas and dhikrs that were written by leading, trustworthy Sunni scholars:
1. al-Wabil as-Sayyib by al-‘Allamah ibn al-Qayyim
2. al-Kalim at-Tayyib by Shaykh al-Islam Ibn Taymiyah
3. al-Adhkar by al-Imam an-Nawawi
These books do not contain any innovated dhikrs; they contain dhikrs narrated in the Quran and Sunnah. They are sufficient and there is no need for books of myths written by jokers, praise be to Allah.
End quote from Majmoo‘ Fatawa Fadeelat ash-Shaykh Salih ibn Fawzan, 2/697
You could also refer to what is quoted on our website in the answer to question no. 13506, where we discussed the conditions of du‘a being answered and the means of having one’s supplication accepted by Allah, may He be exalted.
And Allah knows best.