Mu'adh bin Zuhrah (may Allah have mercy on him) said that it reached him that when the Prophet (peace and blessings be upon him) broke his fast he would say: 'Allahumma laka sumtu wa 'ala rizqika aftartu' ("O Allah, for You I have fasted, and with Your provision I have broken my fast.")
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8134