פּירוש אויף שמות 26:15

ועשית את הקרשים AND THOU SHALT MAKE THE BOARDS — It should be said: “Thou shalt make boards” (without the definite article), the same form which is used of each of the other things (parts of the Tabernacle)! What is the force of the term, “the boards?” It speaks of those boards that already existed as being designated for that purpose. For our father Jacob had planted cedars (shittim-trees) in Egypt and when he died be bade his sons carry them up with them when they would leave Egypt. He told them that God would once command them to erect a dwelling of shittim-trees in the wilderness. “See” — added he — “that they be ready in your possession!” (cf. Rashi on Exodus 25:5). That is what the Babylonian embodied in his liturgical composition: The plant of those who had been admonished by their father Jacob grew up rapidly to become the cedars for the beams of our House (the Tabernacle), which refers to the fact that they had been admonished that they (the cedars) should be ready in their possession in advance (before they were needed).

עצי שטים עומדים, “made of acacia wood, upright.” The reason why he Torah added the word: “upright,” was to forbid planks that had already fallen off the tree after the branch having been sawed off. This comes to teach you that it is proper to build a building only from wood that came from a tree that did not produce any fruit.

עצי שטים עומדים, “made of acacia wood standing upright.” One interpretation of the word עומדים, is that they were taken from trees which up to that time had been standing rooted in the earth, i.e. they had not had time to dry out or to begin to rot. An alternate explanation for the word עומדים would be that the boards remained in their prime condition forever.