Commentary for Numbers 16:28

לעשות את כל המעשים האלה [HEREBY YE SHALL KNOW THAT THE LORD HATH SENT ME] TO DO ALL THESE THINGS — i.e. that I have acted by the Divine Command in giving Aaron the High Priesthood, and in making his sons second in rank in the priesthood and Elzaphan prince of the Kohathites.

בזאת תדעון, "Hereby you shall know, etc." Why did Moses think that after all that had occurred during the preceding two years that the Israelites still did not believe that he was a true prophet? Furthermore, why did he have to repeat כי לא מלבי, "that I did not do all these things arbitraily?" The people did not actually accuse Moses of having acted on his own when he appointed Aaron as High Priest, etc. However, they thought that Moses had exploited his good standing in the eyes of G'd to persuade G'd to agree with his personal choices for these positions. Alternatively, the people had thought that Moses had desired for these particular appointments to be made and G'd had read his mind and indulged him by telling him to make these appointments. The people had not disagreed that Aaron was unworthy to be the High Priest. Moses therefore countered that what was about to occur would be proof that he had not even initiated any of these decisions but that all of them had originated with G'd. As to the people's suspicion that in his heart Moses had wished for these people to be appointed to the positions they had been appointed to at the command of G'd, he said כי לא מלבי, "the thought had never occurred to me." The word כי here indicates that Moses felt he did not need to prove that he had acted on his own. All he had to prove was that he had not even entertained any thought of appointing close relatives to high positions such as the position of being High Priest.

כי לא מלבי, the idea to assign the offering of incense to them did not originate with me,