Question: When rising from ruku’, should the imam, the followers (jama’at), and a person performing salat individually say “Rabbana lakal hamd”?
ANSWER
A person who performs salat individually should say both “Sami’allahu liman hamidah” and “Rabbana lakal hamd” while rising from ruku’. (Radd-ul-mukhtar)

If one performs salat by following the imam, one should not utter, “Sami’allahu liman hamidah,” but utter “Rabbana lakal hamd.” If one performs salat individually, one should say both. The same is written in the books Muhit, Tatarhaniyya, and Hidaya. (Fatawa-i Hindiyya)

It is sunnat for the imam as well as for a person performing salat individually to say “Sami’allahu liman hamidah” while straightening up from ruku’. The followers do not say it, but they should say “Rabbana lakal hamd.” (Se’adet-i Ebediyye)

Even if the imam has not said “Sami’allahu liman hamidah,” the followers say “Rabbana lakal hamd.”

According to Imam-i A’zam, the imam does not say “Rabbana lakal hamd.” According to Imam-i Muhammad and Imam-i Abu Yusuf, he says it. In most of the fiqh books, the qawl of Imam-i A’zam is the preferred one. (Radd-ul-mukhtar)

In the madhhabs of Hanbali and Shafi’i, it is sunnat for the imam, the followers, and a person performing salat individually to say “Rabbana lakal hamd.” In the Maliki Madhhab, it is mandub (an act for which there is reward and if omitted there is no sin) for a person performing salat individually and for the followers to utter it. (Al-Fiqh 'Ala al-Madhahib al-Arba'ah)