Laudator Temporis Acti

William Walrond Jackson, Ingram Bywater: The Memoir of an Oxford Scholar, 1840-1914 (Oxford: Clarendon Press, 1917),pp. 69-70:

He did, however, recommend a most interesting treatise — C.G. Cobet's Oratio de arte interpretandi grammatices et critices fundamentis innixa — for which I was very grateful. It contains the famous description of the modern scholars, Qui Graeca carmina pangunt quae neque Graeca sunt neque carmina.

The phrase occurs (with slightly different word order) on p. 46 of Cobet's Oratio (Leiden: W. Hazenberg, 1847), discussing Lucian:

[V]erum res est tam anceps et lubrica bene Graece scribere, ut nemo umquam nec recentiorum, nec veterum, qui antiquiorem sermonem Atticum imitari voluerit, satis sibi ab ridiculis et pudendis erroribus potuerit cavere. Omitto recentiores et aequales nostros, qui carmina Graeca pangunt quae neque Graeca sunt neque carmina.

That is,

But so doubtful and slippery a matter it is to write Greek well, that no one (among either ancients or moderns) who had the desire to imitate the older Attic idiom was ever able to avoid humorous and embarrassing mistakes. I forbear from mentioning those moderns and our contemporaries who compose Greek verse that is neither Greek nor verse.