大乘起信論義記 3.400

3.400

初、言信根本者，真如之法諸佛所師，眾行之源，故云根本。非直懸起信心，亦乃樂念觀察，故云樂念等也。

Regarding the first, 'having faith in the root': the phenomenon of tathatā is that which all Buddhas take as teacher, the source of all practices; hence it is called the 'root.' One does not merely give rise to faith from afar, but also joyfully contemplates and examines it; hence it says 'joyfully contemplating,' and so forth.