The Remains of a Senarius in a Letter of Jerome?

In a letter written by Jerome to Augustine (Ep. 68 in editions of Augustine's letters; Ep. 102 in editions of Jerome), dated to 402 (see Fuerst's monograph on the correspondence), Jerome threatens Augustine via classical and proverbial metaphors:

nunc te currente et longa spatia transmittente nobis debetur otium simulque, ut cum venia et honore tuo dixerim, ne solus mihi de poetis aliquid proposuisse videaris, memento Daretis et Entelli et vulgaris proverbii, quod bos lassus fortius figat pedem.

In a brief but interesting note in Wiener Studien 19 (1897) 317, Karl Schenkl makes a case that the 'proverb' (italicized in the above citation) is not merely a proverb, but is a citation of a poet--the 'remains' or the 'left-overs' ('die Reste') of a senarius. For support, he offers four reasons: the balance of the phrase, its 'poetic coloring', the placing of the words, and the prominent (or 'pronounced') alliteration of fortius figat.

He says that he is not able to unravel whence it comes; Schenkl points out that Erasmus, in his Adagia, refers it to the Greek proverb Atremas bous (Diog. III.9, Arsen. VI.8, Apost. IV.24), but he states that this contributes nothing to its explanation. He theorizes that Jerome found the proverb in a Gnomologium, and that it was probably included in the fragmenta adespota of iambic poets.