What should I say when I catch up with the Imam while he is rising from bowing?

If you enter the mosque and the Imam has already risen from bowing, then you say: "Rabbana wa lakal-hamd (Our Lord, and to You be all praise)," and you do not say the opening supplication (Du'a al-Istiftah) because it is recited at the beginning of the prayer, nor the seeking of refuge (Al-Isti'adha) because its place is before the recitation. The proof for this is the saying of the Prophet, peace and blessings be upon him: "And when he (the Imam) says: 'Sami' Allahu liman hamidah (Allah hears those who praise Him),' then say: 'Rabbana wa lakal-hamd (Our Lord, and to You be all praise).'" If the latecomer catches the Imam in bowing, he does not recite the opening supplication when he stands for the next bowing, but he does seek refuge with Allah from Satan, the accursed, for the recitation. The sum of it is that whoever catches the Imam in a certain position performs the prescribed remembrance for that position.