With the increase in interest in the subject of intertextuality (see e.g. my three books Isaiah Old and New, Psalms Old and New, and Torah Old and New, all published for Fortress) it is useful to study in depth the citations from the Bible that occur in other sources up to the middle ages, not least because sometimes the citations in those sources are more primitive and likely more original than the ones in straight up Biblical papyri. Here is an online source Larry Hurtado points us to, where all such citations are listed and catalogued.
http://www.biblindex.info/
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