Rashi on Deuteronomy 22:12/רש"י על דברים כ״ב:י״ב

1גדלים תעשה לך THOU SHALT MAKE THEE TASSELS [UPON THE FOUR CORNERS OF THY VESTURE] — be they even from a mixture of wool and linen; for this reason Scripture puts them (the prohibition of שעטנז and the command of צצית) in juxtaposition (Yevamot 4a).