Exodus 38:5-7

Young's Literal Translation

⁵ and he casteth four rings for the four ends of the brazen grate -- places for bars;

⁶ and he maketh the staves of shittim wood, and overlayeth them with brass;

⁷ and he bringeth in the staves into the rings on the sides of the altar, to bear it with them; hollow [with] boards he made it.