Many different duas for iftar have been narrated in various Ahadith, some of which are to be recited before iftar, while some are to be recited afterwards.
The dua mentioned in the question is one which is to be recited before iftar as narrated by Hazrat Muath ibn Zehra that the Prophet (SAW) used to recite the following dua when he had iftar:
اللّٰهُمَّ لَكَ صُمْتُ وَ عَلٰی رِزْقِكَ أَفْطَرْتُ (Abu Dawood)
Although the words of the dua are in the past tense, which causes confusion for Non-Arabs, it is part of the Arabic language convention to use the past tense in such situations.
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دلائل:
مشکوٰۃ المصابیح: (کتاب الصوم، 175/1)
''عن معاذ بن زهرة قال: إن النبي صلی الله عليه وسلم كان إذا أفطر قال: اللّٰهم لك صمت و علی رزقك أفطرت. رواه أبو داؤد مرسلاً''.
الھندیۃ: (200/1، ط: رشیدیة)
''و من السنة أن يقول عند الإفطار: اللهم لك صمت و بك آمنت و عليك توكلت و علی رزقك أفطرت''.
Allah the Almighty knows best.
Darul ifta Al-Ikhlas, Karachi.