John Gill Commentary Exodus 5:17

"But he said, Ye are idle, ye are idle: therefore ye say, Let us go and sacrifice to Jehovah." — Exodus 5:17 (ASV)

But he said, you are idle, you are idle
Instead of expressing indignation at the taskmasters, and relieving the officers and the people, he insults them in a flouting sarcastic way, charging them with sloth and idleness; and which, for the certainty of it, or, however, to show how strongly persuaded and fully assured he was of the truth of it, repeats it, and gives the following as a proof of it,

therefore you say, let us go and do sacrifice to the Lord ;
suggesting that it was not so much the service and honour of God they regarded, as that they might have a leisure day from work and labour.