Or HaChaim on Exodus 5:5/אור החיים על שמות ה׳:ה׳

1 ויאמר פרעה הן רבים עתה עם הארץ, Pharaoh said: "now the people in the land are very numerous, etc." Pharaoh meant that in view of the large numbers of people involved it was impossible to convince all of them that they did not face a danger if they did not march to the desert to offer sacrifices to their G'd. This was why he held Moses and Aaron responsible for the work stoppage if it would occur. He directed this accusation exclusively at Moses and Aaron. Pharaoh meant that even a brief work stoppage was a major economic disaster considering the number of people involved.