When should the follower (Ma'mum) say 'Rabbana wa lakal hamd'?

Primary text

The follower should say 'Rabbana wa lakal hamd' immediately upon rising from Ruku'. This is established because the statement of the Prophet, peace be upon him, 'When the Imam says: Sami' Allahu liman hamidah, then say: Rabbana wa lakal hamd,' implies that the follower's statement should follow the Imam's. Since the follower begins rising immediately after the Imam says 'Sami' Allahu liman hamidah,' the follower's recitation of 'Rabbana wa lakal hamd' occurs at that point in the rising.