Question: May Allah be gracious to you. Some people say at the call to prayer: “O Allah, for You I have fasted.” Is there anything wrong with that?
(Allahumma laka sumtu wa bika amantu wa `alayka tawakkaltu wa `ala rizqika aftartu
(‘O Allah, I have fasted for You, believed in You, placed my trust in You, and broken my fast on your provision’))
Shaikh Abdul Azeez bin Abdullah bin Baaz:
There is a weak hadith about it, and if someone says it, there is no problem in that, but it is not authentic. The hadith is weak, but its meaning is correct: one has fasted for Allah and broken the fast for Allah. However, the hadith should not be called a Sunnah. It can be said that the general act of supplication is a Sunnah, the general act of supplication that one does when breaking the fast is a Sunnah, and it is hoped that it will be accepted.
Question: May Allah be gracious to you, I mean before the Fajr Adhaan?
Answer: At the breaking of the fast and during the fast, it is all the same. If someone says it, there is no harm, but it is not a Sunnah. (End)
Source
Benefits from the Fatwa of Shaikh Abdul Azeez bin Abdullah bin Baaz (Rahimahullah):
1- The Dua is indeed proven from a weak Hadith. And not an authentic Hadith.
2- The Shaikh said that there’s no problem in using the Dua. This shows that Duas from weak Ahadith can be used.
3- One of the conditions being that the meaning of the Dua should be good.
4- That Dua should not be claimed to be from the Sunnah, since it’s not proven authentically.
5- The Dua may be accepted because Duas are generally accepted during the time of Suhoor and Iftaar.
(Translated and benefits added by Mohammed bin Thajammul Hussain Manna)
Authentic Duas Below
Saheehus-Seerah 