According to R. Dr. Haym Soloveitchik the term "minhag avoteinu Torah" or "minhag Torah hi" was likely coined by RabbeuRabbenu Yitschak ben Yehuda, a student of Rabbenu Gershom, and the rebbi of Rashi. He is certainly the first known Ashkenazi to use hethe term. [2]
[2] Minhag Ashkenaz ha-Kadmon. Published in Collected Essays II page 49.