The Dua of Iftaar should be made after having Iftaar. Rasoolullah (peace be upon him) would recite at Iftaar,
اللهم لك صمت و على رزقك افطرت
O Allah! I have kept fast for You and with Your sustenance have I opened my fast.
(Abu Dawood, Part 1, pg. 342, Published by Maktaba Rahmaania, Lahore)
Mujadid-e-Islam Hazrat Mulla Ali Qaari (Allah be pleased with him) writes in the annotation of this narration,
و قال ابن الملك : اى : قرأ بعد الافطار
Ibn Malik (Allah be pleased with him) has stated that Rasoolullah (peace be upon him) would read (the Dua) after Iftaar.
(Mirqaatul Mafateeh, Vol. 4, pg. 488, Published by Maktaba Haqqania, Multan)
Allah Ta’aala states in a Hadeeth-e-Qudsi,
Beloved to Me are those servants of Mine who make Iftaar expeditiously.
(Tirmidhi, Part 1, pg. 150, Published by Qadeemi Kutub Khana, Karachi)
Making Iftaar first and then reciting this Dua is quicker compared to making Dua first and then eating, hence in light of this narrative, the former is more beloved to Allah Ta’aala than the latter.
If one recites the Dua at the time of Iftaar before eating, the fast will still be valid.
Allah and His Rasool know best.
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