Adam Clarke on Psalms 105:28

They rebelled not against his word - Instead of ולא מרו velo maru, "they rebelled," some think that a ש shin has been lost from before the word, and that it should be read ולא שמרו velo shamru, "they did not observe or keep his word." Or the words may be spoken of Moses and Aaron; they received the commandment of God, and they did not rebel against it. They believed what he had spoken, and acted according to his orders. It could not be spoken of the Egyptians; for they rebelled against his words through the whole course of the transactions.