Praise be to Allah.
The Du`a ‘Allahumma laka sumtu was ‘ala rizqika aftartu’: Authentic?
Saying Du`a in the words mentioned when breaking the fast was narrated in an inauthentic Hadith which was narrated by Abu Dawud (2358) from Mu’adh ibn Zuhrah (may Allah be pleased with him), who heard that the Prophet (peace and blessings of Allah be upon him) used to say, when he broke his fast, “Allahumma laka sumtu was ‘ala rizqika aftartu (O Allah, for You I have fasted and by Your provision I have broken the fast).”
Reported Du`a when breaking the fast
There is no need for this, because we have the report narrated by Abu Dawud (2357) from Ibn `Umar (may Allah be pleased with him) who said: When the Messenger of Allah (peace and blessings of Allah be upon him) broke his fast, he would say: “Dhahaba ath-Thama’u, wa abtallat al-‘urooq wa thabata al-ajr insha Allah (Thirst has gone, the veins are moist, and the reward is assured, if Allah wills).” This Hadith was classed as sound by Al-Albani in Sahih Abu Dawud.
Du`a while fasting and when breaking the fast is highly recommended
It is recommended for the fasting person to say Du`a whilst he is fasting and when breaking the fast , because of the report narrated by Ahmad (8030) from Abu Hurayrah (may Allah be pleased with him) who said: We said: O Messenger of Allah, when we see you our hearts are softened and we are people of the Hereafter, but when we leave you we are attracted by this world and by our womenfolk and children. He (peace and blessings of Allaah be upon him) said: “If you were always as you are when you are with me, the angels would shake hands with you and they would visit you in your houses. If you did not commit sin, Allah would bring another people who would commit sin so that He could forgive them.” He said: We said: O Messenger of Allah, tell us about Paradise, what is it built of? He said: “Bricks of gold and bricks of silver; its mortar is musk, its pebbles are pearls and rubies, its soil is saffron. Whoever enters it will be happy and will never be miserable, he will abide therein forever and never die. His clothes will never wear out and his youth will never fade. There are three whose Du`a will not be rejected: a just ruler, a fasting person until he breaks the fast, and the prayer of one who has been wronged. It is borne on the clouds and the gates of heaven are opened for it, and the Lord, may He be Glorified and Exalted, says: ‘By My Glory, I will grant you help even if it is after some time.’”
This Hadith was classed as authentic by Shu’ayb Al-Arna’ut in Tahqiq Al-Musnad.
It was also narrated by At-Tirmidhi (2525) with the words, “… the fasting person when he breaks his fast …” This was classed as authentic by Al-Albani in Sahih At-Tirmidhi.
So you may ask Allah for Paradise and seek refuge with Him from the Fire, and you can pray for forgiveness, and offer other Du`as that are prescribed in Islam.
As for the Du`a in this particular form – Ashhadu alla ilaha ill-Allah astaghfir Allah as’aluka al-jannah wa a’udhu bika min an-nar (I bear witness that there is none worthy of worship but Allah, I ask Allah for forgiveness, I ask You for Paradise and I seek refuge with You from the Fire) – we could not find any source for it.
And Allah knows best.