It's possible that Michelet could have got it from Buchner, though I don't know how well known Buchner was by the 1860's. He wrote the play in 1834 and it was published in 1835, republished in 1850 and again in 1879, though it wasn't actually performed until 1902. Buchner incorporated a lot of quotations into his play, but a lot of Saint-Just's dialogue is his own invention, so I'm not sure if he got that particular phrase from a history book/source or just made it up.