[shani]

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]