Or HaChaim on Numbers 16:5/אור החיים על במדבר ט״ז:ה׳

1 בקר וידע ה׳ את אשר לו, "In the morning G'd will make known who is His, etc." The reason Moses said that the proof would come on the morrow and he did not offer to provide the proof immediately may either have been due to the fact that the incense offered every afternoon had already been offered on that day, or it may have been so that they could not accuse him of having chosen a time of day when G'd is not particularly well disposed towards the people. According to the Zohar second volume page 39 the late afternoon is a time when the attribute of Justice is in the ascendancy. In addition, Moses may have wanted to give Korach and associates time to reflect on the enormity of their challenge so that they could desist and repent before dawn. Our sages in Bamidbar Rabbah 18,7 suggest additional reasons why Moses delayed until the next morning.

2את אשר לו ואת הקדוש, "who is His and who is holy, etc." Why did Moses have to repeat this statement (although he changed the wording somewhat)? The Zohar says that the words אשר לו refer to the status of the Levites, whereas the word קדוש refers to the status of the priesthood. If so, we must still understand why Moses had to mention the Levites altogether seeing that Korach had only challenged the appointment of the priests. If you consider that I have explained that Korach took the elders of Reuven in order to buttress his arguments that Moses had ignored the privileges due to people of seniority, we can understand that Korach had argued that the privileges of seniority should be restored to the members of the tribe of Reuven. In that event Moses also had to defend the appointment of the Levites to their respective positions.

3We must also try and understand why Moses repeated that G'd would bring close to Him the one whom He considered holy, i.e. ואת הקדש הקריב אליו, ואת אשר יבחר יקריב אליו. Perhaps we can understand this by analysing the nature of Korach's complaints. Firstly, he claimed that Aaron was not specifically suited to be High Priest seeing there were people more qualified than he. Secondly, Korach argued that even if he granted Aaron's qualifications to be High Priest, there were others equally qualified to perform the sacrificial service with him. Why should he enjoy an exclusive? Concerning the argument that there were people better qualified than Aaron, Moses said את אשר לו, that G'd would demonstrate whom He had chosen, i.e. who had been designated for this task since creation. We have a parallel situation in Jeremiah 1,5 where G'd told Jeremiah that He had selected him for his task already before he was born. The words ואת הקדש mean that after having come into the world Aaron had sanctified himself in order to be worthy to occupy the position for which he had been destined. Moses carefully chose the expression והקריב to make it plain that Aaron had already been divinely approved and that G'd had seen fit to make His שכינה come to rest on him. Concerning the claim that others were just as worthy as Aaron to perform the service in the Tabernacle, Moses said ואת אשר יבחר בו יקריב אליו, "if G'd chooses anyone in addition He will bring him close to Him, i.e. they too will be allowed to enter the Tabernacle, etc."