Or HaChaim on Exodus 29:10/אור החיים על שמות כ״ט:י׳

1 וסמך אהרון ובניו את ידיהם, "and Aaron and his sons will place their hands, etc." The reason that the Torah uses the singular וסמך instead of the plural form וסמכו, is that Aaron was to place his hands on the bullock and his sons were to repeat this action individually, one at a time. I have come across a Tossephta in chapter 10 of Menachot which states: "When five people jointly offer one sacrifice they all perform the rite of placing their hands with all their weight on the animal; they do not do this simultaneously but one after another." Maimonides rules similarly in chapter 3 of Maaseh Ha-korbanot. Accordingly, we can well understand why our verse describes the act of סמיכה in the singular, i.e. וסמך, instead of וסמכו, as they were not to do this simultaneously. We still have to clarify why the Torah had to give these instructions twice in connection with the ram in verse 19 and was not content with Leviticus 8,18 where it is reported that Aaron and his sons did perform סמיכה and the fact is reported in the plural. Perhaps the Torah felt that if it would not repeat the report we would think that seeing that the bullock was a sin-offering, only Aaron performed the rite of סמיכה because only he and not his sons had been involved in the sin of the golden calf. By telling us that the same procedure was followed in connection with the ram which was a total-offering, the Torah made plain that none of these offerings were connected with Aaron's share in the making of the golden calf. The method chosen by the Torah makes it clear that the performance of סמיכה was required for all meat-offerings other than birds. Now we can understand why there was no need to write the word ויסמכו in the singular in Leviticus 8,18, seeing the point had been made already in our verse here. [With all due respect to the author, why did the Torah repeat the point in Leviticus 8,14 where the Torah again uses the singular in connection with Aaron and his sons? Ed.]