Or HaChaim on Exodus 32:9/אור החיים על שמות ל״ב:ט׳

1 ויאמר ה׳ אל משה, G'd said to Moses, etc. The reason this is reported as a separate address by G'd to Moses (seing there had been no interruption since G'd commenced to speak in verse 7) is that the first address was only an explanation of why Moses had to descend from his lofty spiritual niveau. Now G'd speaks to Moses concerning the repercussions Israel's conduct will have on their own standing. Had the Torah not inserted the words ויאמר ה׳ at this point, we would have thought that the previous sentence "they have quickly departed, etc." was meant as an introduction to why the people would be punished.