Or HaChaim on Numbers 17:6/אור החיים על במדבר י״ז:ו׳

1 וילנו כל עדת בני ישראל, The whole congregation of Israel murmured, etc. What is the meaning of the word לאמר in this verse? Seeing the complaint was directed only at Moses and Aaron as has been stated what could the word לאמר possibly add? Perhaps the people did not spell out their complaint as did the Torah., i.e. they did not actually say: "you have killed the people of G'd." They merely voiced complaints which amounted to much the same as if they had made this direct accusation which the Torah summarised here. The words וילנו לאמר read together were equivalent to their having said the words quoted in their name in our verse.

2Furthermore, the audacity of the people who had witnessed how Moses had not stood on ceremony but had himself gone to Datan and Aviram in order to give them a chance to escape death if they recanted, is impossible to understand. What could Moses and Aaron possibly have been guilty of? We must conclude therefore that the complaint of the people concerned the very type of test Moses and Aaron arranged, i.e. the offering of incense of all the associates of Korach at one and the same time. They claimed that Moses only had to make this test between Korach and Aaron and as a result, as soon as G'd had shown that He accepted the incense from Aaron and not from Korach, the other 250 men would have learned their lesson and would not have had to die by engaging in this confrontation. The word לאמר was an accusation against Moses who had made all these men offer incense. The people did not complain about the death of either Korach or Datan and Aviram. They were only angry at the death of the 250 men who represented the elite of the nation, its most senior judges, etc., who they claimed had died through an exercise orchestrated by Moses and Aaron. According to some of our sages, the people's complaint was directed at the very nature of the offering Moses had insisted on, i.e. קטורת, the holiest of the holy. He should have allowed them to perform a relatively minor form of Temple service instead. According to that view the word לאמר addresses the instruction to offer incense.