Is it mandatory to say bismillah before breaking the fast in Ramadan? I usually just say a small dua (allahumma laka sumtu wa bika aamantu wa`ala rizq-ika aftartu) before I do, is this permissible?
Saying "Bismillah بسم الله " is not mandatory, but among the sunan of eating.
This is based on:
"When one of you eats food, then let him say: 'Bismillah.' If he forgets in the beginning, then let him say: 'Bismillah Fi Awwalihi Wa Akhirih (In the Name of Allah in its beginning and its end.)"
It is also permissible to say the full basmalah:
Bismiallah ar-Rahmani ar-Raheem
بسم الله الرحمن الرحيم
Having this said this ruling applies at any meal not necessarily nor exclusively for iftar in Ramadan.
Further there's hardly a n authentic du'a for breaking the fast as you may read in: What Duaa is to be recited before breaking the fast and at the beginning of the fast?