It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) (the narrator thinks it is narrated from the Prophet (peace and blessings be upon him)) that when he (peace and blessings be upon him) raised his head from bowing (ruku'), he would say: "Allahumma rabbana laka al-hamd, mil'a al-samawat, wa mil'a al-ard, wa mil'a ma shi'ta min shay'in ba'd (O Allah, our Lord, to You belongs all praise, filling the heavens, filling the earth, and filling whatever else You will thereafter)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12450