妙法蓮華經玄義 8.40

8.40

心是行本者，《大集》云：「心行、大行、遍行」。心是思數，思數屬行陰，諸行由思心而立，故心為行本。

As for the mind being the root of practice, the Mahāsaṃnipāta says: "Mental practice, great practice, all-pervading practice." The mind is the factor of volition (cetanā); volition belongs to the aggregate of formations. Since all practices are established through the volitional mind, the mind is the root of practice.