Musar for Zevachim 230:6
עד שלא הוקם המשכן [וכו']: יתיב רב הונא בר רב קטינא קמיה דרב חסדא וקא קרי (שמות כד, ה) וישלח את נערי בני ישראל א"ל הכי אמר ר' אסי (קרבו) ופסקו
son of R'Kattina sat before R'Hisda, and recited [the text], And he sent the young men of the children of Israel, [who offered burnt-offerings, and sacrificed peace-offerings of oxen unto the Lord].<span class="x" onmousemove="('comment',' Ex. XXIV, 5. The 'young men' were the firstborn, not priests, and the occasion was when Moses built an altar at the foot of Mount Sinai (ibid. v. 4) .');"><sup>6</sup></span> Said he to him: Thus said R'Assi: And then they ceased.<span class="x" onmousemove="('comment',' This was the last time that the firstborn performed the sacrificial service, though it was nearly a year before the Tabernacle was set up.');"><sup>7</sup></span>