A Relationship Between the Gospel of Thomas and the Gospel of the Hebrews: "It seems that, in constructing his 'harmony' [Diatessaron], he [Tatian] used not just the 'canonical' Gospels, but also a 'fifth source', whose material sometimes agreed with readings proper to texts like the Gospel to the Hebrews or the Gospel of Thomas. It has been proposed to identify this source with that Aramaic tradition of Palestinian origin, independent of the Greek Gospels, that had first conveyed a knowledge of Jesus to the peoples of Syria, pivoting, to begin with, on the region's Jewish communities. Certainly, West Aramaic expressions or, more generally, elements left a deep mark not just and perhaps not so much on the Peshitta as on the more general exegetical and 'spiritual' meditation of the Churches of Syria, as many recent studies have demonstrated (cf. e.g. S. Brock, The Lost Old Syriac...).
Paolo Bettiolo, Syriac Literature, page 426: https://archive.org/details/SyriacLiterature
