Exodus 28:31

The robe of the ephod — This was next under the ephod, and reached down to the knees, without sleeves, and was put on over their head, having holes on the sides to put the arms through, or, as Maimonides describes it, was not sewn together on the sides at all. The hole on the top through which the head was put was carefully bound about, that it might not tear in the putting on. The bells gave notice to the people in the outer court, when he went into the holy place to burn incense, that they might then apply themselves to their devotions at the same time, Luke 1:10, in token of their concurrence with him, and their hopes of the ascent of their prayers to God in the virtue of the incense he offered. Aaron must come near to minister in the garments that were appointed him, that he die not. 'Tis at his peril if he attend otherwise than according to the institution.