When beginning the fast- Seher
وَبِصَوْمِ غَدٍ نَّوَيْتَ مِنْ شَهْرِ رَمَضَانَ
Wa bisawmi ghadinn nawaiytu min shahri ramadan
I intend to keep the fast for tomorrow in the month of Ramadan
[abu Dawud]
When breaking the fast- Iftar
اللَّهُمَّ اِنِّى لَكَ صُمْتُ وَبِكَ امنْتُ [وَعَلَيْكَ تَوَكَّلْتُ] وَعَلَى رِزْقِكَ اَفْطَرْتُ
Allahumma inni laka sumtu wa bika aamantu [wa 'alayka tawakkaltu] wa 'ala rizq-ika aftarthu
O Allah! I fasted for You and I believe in You [and I put my trust in You] and I break my fast with Your sustenance
["wa 'alayka tawakkaltu" is quoted in some books of knowledge - but not all, hence it is in brackets]
[abu Dawud]