1 Corinthians 9:13

Do ye not know that they which minister about holy things live of the things of the temple? and they which wait at the altar are partakers with the altar? (KJV)

Then: Paul's question compares the normal set-up of Jewish leaders being supported in the temple with the work of apostles for Christ. If Israelite priensts received tithes and sacrifices, should not the same practice be followed in the churches - Corinth included? Paul believes it should. Fee (p. 411-412) sees the verse as an interruption or introduction to Paul's final thoughts, verse 12, when he suddenly justifies himself with two more reasons. He uses two analogies that are much closer to Jewish religious practices. Both the Levites and the priests eat the temple sacrifices and use the money given in offering.

Now: Paul's reasoning set the on-going policy throughout the centuries for the way pastors and church leaders are paid; but for the most part, ministers do not follow his example of getting second jobs just to be in ministry.

Extra source: The First Epistle to the Corinthians by Gordon D. Fee (Eerdman's 1987) online at http://books.google.com/books?id=XlBp10nUTXAC&dq=Gordon+Fee+Bible+commentator&printsec=frontcover&source=in&hl=en&ei=SwLNSu3nA4v6Mf2XlDo&sa=X&oi=book_result&ct=result&resnum=12#v=onepage&q=&f=false