Tafsir of Ash-Shu`ara' 26:110

The fa (ف) in His saying, the Exalted, "So fear Allah and obey me," serves to sequence what follows it upon what preceded it, regarding his—peace be upon him—exoneration from greed. Just as its predecessor was for sequencing what followed it upon his being a Messenger from Allah, the Exalted, containing the benefit of both abodes, along with his trustworthiness.

The repetition is for emphasis and to alert [the listener] that each of the two [states] is independent in necessitating piety and obedience, so how much more so when they are combined?

It was recited, "My reward" (ajriya), with the ya being quiescent (sukūn). Both the quiescent ya and the fatha are two well-known linguistic variants in such cases, and the grammarians have differed as to which of the two is the original form.