Albert Barnes Commentary Exodus 2:13

"And he went out the second day, and, behold, two men of the Hebrews were striving together: and he said to him that did the wrong, Wherefore smitest thou thy fellow?" — Exodus 2:13 (ASV)

Your fellow - “ Your neighbor.” The reproof was that of a legislator who established moral obligations on a recognized principle. Hence, in the following verse, the offender is represented as feeling that the position claimed by Moses was that of a Judge. The act could only have been made known by the Hebrew on whose behalf Moses had committed it.