Sunan Abi Dawud, Book 13, Hadith 2351 (USC-MSA) ⓘ
Narrated Mu'adh ibn Zuhrah: The Prophet of Allah used to say when he broke his fast: O Allah, for Thee I have fasted, and with Thy provision I have broken my fast.
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا هُشَيْمٌ، عَنْ حُصَيْنٍ، عَنْ مُعَاذِ بْنِ زُهْرَةَ، أَنَّهُ بَلَغَهُ أَنَّ النَّبِيَّ صلى الله عليه وسلم كَانَ إِذَا أَفْطَرَ قَالَ " اللَّهُمَّ لَكَ صُمْتُ وَعَلَى رِزْقِكَ أَفْطَرْتُ " .
also referenced under: Hadith 2358 (DarusSalam) & Hadith 2351 (Hasan)