How should the ma'mum (one praying behind the imam) perform the takbir (saying Allahu akbar) and his supplication behind the imam?

Question:
What is obligatory for the ma'mum (one praying behind the imam) when the imam says, "Allahu akbar," and "Sami' Allahu li man hamidah" (Allah hears the one who praises Him)? Should they respond with a loud voice or in a low voice, whether the prayer is one of silence or one of loud recitation? This questioner is from Yemen.

Answer:
When the imam says, "Sami' Allahu li man hamidah" (Allah hears the one who praises Him), the ma'mum (one praying behind the imam) says, "Rabbana wa laka al-hamd" (Our Lord, to You belongs all praise). There is no need to raise the voice; he should say words that he can hear himself: "Rabbana wa laka al-hamd" (Our Lord, to You belongs all praise). This is based on the statement of the Prophet (peace and blessings be upon him): Thus, the Prophet (peace and blessings be upon him) commanded it. The imam says, "Sami' Allahu li man hamidah" (Allah hears the one who praises Him), and the ma'mum (one praying behind the imam) says, "Rabbana wa laka al-hamd" (Our Lord, to You belongs all praise).
As for the ma'mum (one praying behind the imam), when he raises his hands, he says, "Rabbana wa laka al-hamd" (Our Lord, to You belongs all praise), or "Allahumma Rabbana laka al-hamd" (O Allah, our Lord, to You belongs all praise), or "Rabbana laka al-hamd" (Our Lord, to You belongs all praise). All of these are permissible. Alhamdulillah, yes.

The presenter: Praise be to Allah.