Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.132.

Ibn Majah related from ‘Abdullah ibn ‘Amr ibn al-‘Aas that the Prophet, upon whom be peace, said: “A fasting person, upon breaking his fast, has a supplication that will not be rejected. When ‘Abdullah broke his fast he would say: “O Allah, I ask of You, by Your mercy that encompasses everything, to forgive me.”

It is confirmed that the Prophet would say: The thirst has gone, the glands are wet and, Allah willing, the reward is confirmed. In mursal form, it is reported that he would say: “O Allah, for You I have fasted and with Your provisions do I break my fast.”

At-Tirmizhi recorded, with a good chain, that the Prophet said: “Three people will not have their supplications rejected: a fasting person until he breaks his fast, a just ruler, and an oppressed person.”