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.