What are the supplications mentioned in the Sunnah that Muslims should say while breaking the fast and while fasting?
In this fatwa:
You may recite any of the following supplications (duaa) when you break your fast:
Allaahumma laka sumtu wa alaa rizqika aftartu
or
Dhahaba al-ddhama’ wa ibtallati al-‘urooq wa thabata al-ajru in sha’ Allah
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 Prophet Muhammad (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 would say: “The thirst has gone, the veins become wet and, Allah willing, the reward is confirmed.”
In another (mursal) narration, the Prophet would say: “O Allah, for You I have fasted and with Your provisions do I break my fast.”
At-Tirmidhi reported 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.”
Moreover, Sheikh Ahmad Kutty, A Senior Lecturer and an Islamic Scholar at the Islamic Institute of Toronto, Ontario, Canada, adds:
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.
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).