Via aish.com… Threadbare:
The good news is that there is a clear and early source that mentions tying a red string to ward off an “evil eye” and that is in the Tosefta, an early Talmudic work (Shabbat, ch. 7-8). The bad news is that it clearly states that tying a red string around oneself is severely prohibited. It is characterized as “Darchei Emori,” a worthless, superstitious practice, close to idol-worship.