It is common knowledge that the apostle knew by heart the entire text of the Hebrew Scriptures. He also was able to cite another version at will: the Greek version of the Bible known as the Septuagint. This is the version he almost always quote in his letters to Greek-speaking Christians.
Thus: when he quoted from the Scriptures, he didn’t have to look it up.
Just ran across this tradition concerning the rabbi Shammai, the important theologian who lived in the first century BC, that is, a couple of generations before Paul. He affirmed that in effect he owned two copies of the Bible:
There was the incident of a certain gentile who came before Shammai. He said to him, “How many Torahs do you have?” [Shammai] said to him, “Two, one in writing, one memorized.” [b. Shabbat 31A, Babylonian Talmud, Neusner edition, 2:127]
Two copies of the Bible, true for Shammai, true for Paul. How true is this for us?
“Paul had the Bible memorized!” by Gary S. Shogren, Seminario ESEPA, San José, Costa Rica
Open Our Eyes, Lord! 
Odd that Neusner renders תורה שבעל פה as ‘memorized [Torah].’ Everyone I’ve ever read takes תורה שבעל פה as the Oral Torah in contrast to the written text.
What is meant by “common knowledge”? Evidence?
Thanks
Gus
Rabbis of that period memorized the Hebrew Scriptures by heart; it is a solid inference that Saul, disciple of Gamaliel, followed the common practice.
People in the first century could not whip out their Bibles to check on a quotation – much of their dialogue and exposition was done from memory, either quoting exactly or paraphrasing the idea.
http://www.stevecorn.com/2010/11/jewish-educational-system/
Interesting!
Hey there! Great to hear from you, blessings!