ANSWER:
The du’a “Allahumma laka sumtu wa ‘ala rizqika aftartu (O Allah, for You have I fasted and by Your provision I have broken my fast)” was narrated by Abu Dawud, 2358. It is a mursal hadith so it is da’if (weak). Al-Albani classed it as da’if in Da’if Abi Dawud (510)
‘Umar said: The Messenger of Allah (peace and blessings of Allah be upon him) used to say when breaking his fast: “Dhahaba al-zama wa’btalat al-‘uruq wa thabata al-ajr in sha Allah (Thirst is gone, the veins are moistened and the reward is certain if Allah wills).” (Narrated by Abu Dawud, 2357; al-Daraqutni, 25. Ibn Hajar said in al-Talkhis al-Habir (2/202): al-Daraqutni said, its isnad is sahih.)