Exodus 16:34 - Matthew Poole's English Annotations on the Holy Bible

i.e. Before the ark, which is called the ark of the testimony, Exodus 25:16; and here, by way of abbreviation, the testimony, or witness, because in it were the tables of the covenant, or the law of God, which was a testimony of God's authority and will, and of man's subjection and duty, or of the covenant made between God and man. See Deuteronomy 10:5, Deuteronomy 31:26. Quest. How could this be laid up before the ark, when the ark was not yet built? Answ. This text only tells us that Aaron did lay it up, but it doth not determine the time, nor affirm that it was done at this instant, but rather intimates the contrary, and that it was done afterwards when the testimony, i.e. the ark, was built. As the next verse also speaks of what was done in the following forty years.