The Prophet, peace and blessings be upon him, said:
“Three du’as are never rejected: the du’a of the father, and the du’a of the one who is fasting, and the du’a of the traveller.” (Recorded al-Bayhaqi)
In another narration, it is reported that he, peace be upon him, said:
“Three people’s du’as are never rejected: the one who is fasting, until he breaks his fast; the just ruler; and the one who has been wronged. Allah raises it above the clouds, and the doors of the skies are opened for it, and the Lord says, ‘By My Honour and Glory! I will help you, even if it be after some time!” (Recorded in al-Tirmidhi)
These hadith shows us that the one who is fasting can make du’a at any time throughout the day, and his du’a will be responded to.
In a third hadith, the time just before breaking the fast has been specifically mentioned, so a person should strive to make sincere du’a at this time also. The Prophet, peace and blessings be upon him, said:
“Indeed the one who is fasting has – while he is about to break his fast – a du’a that is not rejected.” (Recorded in al-Tirmidhi)