Albert Barnes Commentary Numbers 15:32

"And while the children of Israel were in the wilderness, they found a man gathering sticks upon the sabbath day." — Numbers 15:32 (ASV)

Moses mentions here, as is his custom , the first open transgression and its punishment in order to exemplify the laws which he is laying down. The offence of Sabbath-breaking was one for which there could be no excuse. This law at least might be observed even in the wilderness. Transgression of it was therefore a presumptuous sin, and was punished accordingly.