As-salamu ‘alaykum. I understand that saying “rabbana laka l-hamd” is fard after rising from ruku‘ according to some scholars. But I read in some sahih hadiths that Nabiyy salla llahu ‘alayhi wa-s-sallam sometimes repeated the phrase “li-rabbi l-hamd” after rising from ruku‘. If I say that and not “rabbana laka l-hamd” would it fulfill the obligation, and can I say this in both nafl and fard prayers?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

This wording which is “Li Rabi al-Hamd” fulfills what is required, there is no difference between this wording and “Rabbana wa lakal-Hamd” for it is reported by the Prophet . Ibn al-Qayyim said in Zaad al-Ma'ad speaking about the guidance of the Prophet : “And when He (the Prophet ) stands upright [i.e., after rising from Rokoo’] He says: “Rabbana wa lakal-Hamd”, and perhaps He says: “Rabbana lakal-Hamd”…and it was authentically reported that he repeated his saying: “Li Rabi al-Hamd”, li Rabi al-Hamd, until it was as far as bowing.”

There is no difference between obligatory and supererogatory prayers in this ruling.

Allah knows best.