This dua should be recited just before breaking the fast
للَّهُمَّ لَكَ صُمْتُ
Allahumma laka sumtu
وَعَلَى رِزْقِكَ اَفْطَرْتُوَ
wa `ala rizqika aftartu
وَ عَلَيْكَ تَوَكَّلْتُ
wa alayka tawakkaltu
'Dua-e-Iftaar' is tagged in Ramadhan collection. See other dua's from Ramadhan