It is narrated from Hudhayfah (may Allah be pleased with him): I prayed with the Messenger of Allah (peace and blessings be upon him) in the month of Ramadan. When he (peace and blessings be upon him) raised his head from bowing, he said: 'Sami'allahu liman hamidah, Allahumma rabbana wa lakal-hamd' (Allah has heard the one who praised Him. O Allah, our Lord, Yours alone is the being worthy of all praise).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2613