Charles Spurgeon Commentary Matthew 26:73

"And after a little while they that stood by came and said to Peter, Of a truth thou also art [one] of them; for thy speech maketh thee known." — Matthew 26:73 (ASV)

Even when Peter swore, there was something of the Galilean accent in his speech, so that these people in Jerusalem detected his provincial dialect and said to him, Surely thou also art one of them; for thy speech betrayeth thee. If a child of God begins to swear, he will not do it as the ungodly do, and he will be sure to be found out.