Rashi on Exodus 29:7/רש"י על שמות כ״ט:ז׳

1ומשחת אתו AND THOU SHALT ANOINT HIM — This anointing also, was in the form of a X: he put a drop of oil on his head and another drop between his eyebrows and joined them with his finger into this shape (Keritot 5b).