A Muslim should be keen to follow the Sunnah in all aspects of his life, let alone in acts of worship. While fasting, a Muslim should be constant in making dhikr, doing good deeds, etc.
In his well-known book, Fiqh As-Sunnah, the late Sheikh Sayyed Sabiq states: Ibn Majah reported from `Abdullah ibn `Amr ibn al-`Aas that the Prophet (peace and blessings be upon him) 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 (peace and blessings be upon him) would say: “The thirst has gone, the veins become wet and, Allah willing, the reward is confirmed.”
In another (mursal) narration, it is reported that he (peace and blessings be upon him) would say: “O Allah, for You I have fasted and with Your provisions do I break my fast.”
At-Tirmidhi reported that the Prophet (peace and blessings be upon him) said: “Three people will not have their supplications rejected: a fasting person until he breaks his fast, a just ruler, and an oppressed person.”
Moreover, Sheikh Ahmad Kutty, A Senior Lecturer and an Islamic Scholar at the Islamic Institute of Toronto, Ontario, Canada, adds: You may recite any of the following supplications when you break your fast: Allaahumma laka sumtu wa alaa rizqika aftartu
(O Allah! I fasted for Your sake and I am breaking my fast with the food You have provided).
Dhahaba al-ddhama’ wa ibtallati al-‘urooq wa thabata al-ajru in sha’ Allah
(The thirst is gone and the arteries are supple and wet, and reward is complete by the will of Allah).