Where is Hebrews 3:7 quoted from?

Les explains that Paul wrote the book of Hebrews primarily to Jewish believers, so he frequently uses Old Testament contacts and experiences to make his point. In Hebrews 3:7, Paul quotes from the book of Psalms, saying, 'Wherefore, as the Holy Spirit saith, Today if you will hear his voice, harden not your hearts, as in the provocation, as in the day of temptation in the wilderness, when your fathers tempted me, proved me, and saw my works forty years.' Les points out that this refers to the forty-year wilderness experience that followed Israel's rejection of going into the Promised Land at Kadesh Barnea. That generation grieved God, and they feared losing their women and children.