Tractate Pesachim - Chapter 5 - Mishnah 5 פסחים פרק ה׳ משנה ה׳

The pesah is slaughtered in three divisions, as it is said, “And the whole assembly of the congregation of Israel shall slaughter it” (Exodus 12:6): “assembly,” “congregation,” and “Israel.” The first division entered, the Temple court was filled, and they closed the doors of the Temple court. They sounded a teki'ah, a teru'ah, and a teki'ah. The priests stood in rows, and in their hands were basins of silver and basins of gold, a row which was entirely of silver was of silver, and a row which was entirely of gold was of gold, they were not mixed. And the basins did not have flat bottoms, lest they put them down and the blood becomes congealed.