The quote is from Robin Scroggs in The Interpreter’s Dictionary of the Bible, Supplementary Volume, Abingdon Press, Abingdon, Nashville, 1976, page 966

Ronald Fung, says Scroggs is “probably right” about it being a baptismal formula, and F. F. Bruce cites it as a possibility.

Ronald Fung The Epistle to the Galatians (The New International Commentary on the New Testament) Eerdmans, Grand Rapids, Michigan, 1988, page 175

F. F. Bruce The Epistle to the Galatians (The New International Greek Testament Commentary) Paternoster Press, Exeter, 1982, page 187