Rashi on Exodus 15:18/רש"י על שמות ט״ו:י״ח

1לעלם ועד The word ועד is an expression denoting ETERNITY — The ו in it is a root-letter, and consequently it is vowelled with a Patach; but in the similar word in (Jeremiah 29:23) “I am He that knoweth and am witness (ועד)”, where the ו is a servile letter (a prefix) it is vowelled with a Kametz.