Reference: Sharh ‘Umdatil-Fiqh – Tape No.10, Question No.26
Question: Upon rising from the rukoo’, is it permissible for the one following the prayer [behind the imaam] to utter «رَبَّنَا وَ لَكَ الْحَمْدُ حَمْدًا كَثِيرًا طَيِّبًا مُبَارَكًا فِيهِ»?
Response: Yes. If he is able to, and there is sufficient time for him [to do so], then he can utter this, otherwise it is sufficient [for him to simply utter] «رَبَّنَا وَ لَكَ الْحَمْدُ». If, however, he is unable to do so, then this is sufficient, and that is [the minimum] which is required, while if he were to utter more than this, then that is better.