John Gill Commentary Exodus 8:1

"And Jehovah spake unto Moses, Go in unto Pharaoh, and say unto him, Thus saith Jehovah, Let my people go, that they may serve me." — Exodus 8:1 (ASV)

And the Lord spoke unto Moses
Either whilst the plague upon the waters continued, or immediately upon the removal of it:

go unto Pharaoh, and say unto him, thus says the Lord, let my
people go, that they may serve me ;
mentioning neither time nor place, where, when, and how long they should serve him, for which their dismission was required, but insist on it in general.