Hadith 5:38
Mu'adh b. Zuhra told that when the Prophet broke his fast he said, “O God, for Thee I have fasted and with Thy provision I have broken my fast.” Abu Dawud transmitted it in mursal form.
وَعَنْ مُعَاذٍ بْنِ زُهْرَةَ قَالَ: إِنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَانَ إِذَا أَفْطَرَ قَالَ: «اللَّهُمَّ لَكَ صَمْتُ وَعَلَى رِزْقِكَ أَفْطَرْتُ» . رَوَاهُ أَبُو دَاوُد مُرْسلا
Original Reference: Mishkat al-Masabih 1994