In Surah Kausar, Allah states Fasalli Lirabbika. What is its meaning and to whom is it addressed? How does it differ from the command of Aqeem al-Salat which appears multiple times directed towards the believers?
Sura Kauthar is addressing the Prophet Muhammad (SAWA) himself informing him about the great blessed bounties granted to him ( Lady Fatima ) and refuting his enemies' claim that he is a cut off person (Abtar means with no issue). Fasalli Lirabbika means : So, Pray to Your Lord (Who Granted you the Kauthar).
Aqim Assalat أَقِم الصلاة (Establish Prayers) is the order to all believers while Sura Al-Kauthar is addressing the Prophet Muhammad (SAWA) alone.
Wassalam.