This is not a quote from any ancient text. The Israelites were perfectly capable of composing their own poetic literature and there is no reason to assume that this is copied from an ancient text. There are indeed a few quotes from ancient books in the bible but they are usually introduced (e.g., Num. 21:27) and sometimes even given a source (ibid verse 14)!

In fact, biblical critical scholarship usually looks for possible influences of ANE literature on biblical compositions, but it almost never looks or finds any direct parallels between old ANE and biblical texts, it is rather common motifs, themes; shared myths, legends, legendary figures, etc. that they are after. There are a few famous direct literary parallels between biblical and ANE texts, e.g., Proverbs 22:17-24:22, which draws on an Egyptian wisdom text (the Instruction of Amenemope), and Psalm 104 which was directly influenced from the Great Hymn to the Aten (you can see [here][1] for more), but these are exceptions rather than the rule! Direct borrowing of language and parallels is more common between the biblical prophetic books themselves, but it is rare to find a direct literary parallel, never mind a quote, from an ancient ANE text in the OT.   

In any case, I'm not aware of any direct borrowing of language in the book of Job. But it is true that the book of Job was influenced by Ancient Wisdom texts and a lot of its motifs are drawn from ancient Egyptian and Mesopotamian texts. In chapter 8 the only foreign influence I was able to find is verses 8-10 which contends that wisdom from the ancients is significant. This is also a common theme in ancient Sumerian proverbs and the wisdom literature from Mesopotamia. But there are no direct literary parallels to be found in this chapter, and, needless to say, no quotes from ancient texts.  





  [1]: http://www.bibleodyssey.org/places/related-articles/egyptian-influences-on-the-biblical-text