"I said, I will take heed to my ways, that I sin not with my tongue: I will keep my mouth with a bridle, while the wicked is before me." -Psalm 39:1, KJV

James chapter 3 deals with this subject more thoroughly, but the principle is clear here. Sinning with our tongues is easily the easiest sin to fall into as Christians. James tells us that those who can control their tongues have mastered the whole body and are perfect--meaning it cannot be done here on earth! However, even though we cannot master sinless perfection here on earth, God gives us grace to improve as we draw closer to Him.