Du’a When Breaking Fast
Question

What is the du’a that we should say when breaking our fast?
Praise be to Allah.Du’a for breaking fast: (Dhahaba al-zama…)
‘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.)
Allahumma laka sumtu…: Is this dua for breaking fast authentic?
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) 
Du’a after acts of worship
Saying du’a after doing acts of worship is deeply rooted in the Shari’ah, such as making du’a after praying and after completing the rituals of Hajj. Fasting is not excluded from that in sha Allah. Allah mentioned the verse of du’a and encouraged du’a among the verses that speak of fasting, as Allah says (interpretation of the meaning): 
“And when My slaves ask you (O Muhammad) concerning Me, then (answer them), I am indeed near (to them by My Knowledge). I respond to the invocations of the supplicant when he calls on Me (without any mediator or intercessor). So let them obey Me and believe in Me, so that they may be led aright.” [al-Baqarah 2:186]
Importance of making du’a in Ramadan 
Pointing out the importance of making du’a during Ramadan, Shaykh al-Islam Ibn Taymiyah said: 
“Allah tells us that He is close to His slaves and responds to the supplication (du’a) of the one who calls upon Him. Here He is telling us of His Lordship (rububiyyah) over them and that He gives them what they ask for and He responds to their du’a. For if they call upon Him that means they believe that He is their Lord… Then He enjoins two things upon them, as He says (interpretation of the meaning): 
“So let them obey Me and believe in Me, so that they may be led aright.” [al-Baqarah 2:186]
The first is that they should obey His commands to worship Him and seek His help. 
The second is that they should believe in His Lordship (rububiyyah) and Divinity (uluhiyyah), and that He is their Lord and their God. Hence it is said that response to a person’s du’a is indicative of the soundness of his belief and the perfection of his obedience, because the verse of du’a ends with the words (interpretation of the meaning): 
“So let them obey Me and believe in Me, so that they may be led aright.” [al-Baqarah 2:186] (Majmu’ al-Fatawa, 14/33)
For more, please see these answers: 93066 , 14103 , and 65955 .
And Allah knows best.