Reference for Taanit 4:14

הרי מ"ם יו"ד מ"ם הרי כאן מים מכאן רמז לניסוך המים מן התורה

R'Akiba says: On the sixth day of the Feast. R'Judah says in the name of R'Joshua: The last to step before the Ark on the last day of the Feast makes mention, the first does not; on the first day of Passover the first makes mention, the last does not.

Jastrow