Albert Barnes Commentary Matthew 23:29

"Woe unto you, scribes and Pharisees, hypocrites! for ye build the sepulchres of the prophets, and garnish the tombs of the righteous," — Matthew 23:29 (ASV)

Ye build the tombs of the prophets. That is, you build sepulchres or tombs over the prophets who have been slain. This they did professedly from veneration and respect for their character. This is often done in the East in the present day, and indeed elsewhere. Among Muslims, it is a common way of showing respect for any distinguished man to build a tomb for him. By doing this, they profess respect for his character and veneration for his memory. So the Pharisees, by building tombs in this manner, professedly approved of the character and conduct of the prophets and disapproved of the conduct of their fathers in killing them.

And garnish, etc. That is, adorn or ornament. This was done by rebuilding them with more taste, decorating them, and keeping them neat and clean. The original word also means to show any proper honour to the memory of the dead, such as by speaking well of them, praying near them, or building synagogues near to them in honour of their memory.