Allaahumma laka sumtu wa ala Rizqika Aftartu. And I was looking for any authentic hadith for above dua but did not find one.
بسم الله الرحمن الرحيم
As far as we know there is no hadith regarding the dua of sawm before sehri. Yes, there are both types of duas for iftar time dua are mentioned in hadith. The Ulama have explained that the Prophet (pbuh) used to read اللهم لك صمت الخ before iftar and ذهب الظمأ الخ after iftar.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.