俱舍論頌疏論本 1.131

1.131

此即名意處及七界者，立處界門。此識蘊於十二處中即名意處，於十八界門名七心界。於六識外更加意界，名七心界。

"This is called the mind base and [should be known as] seven elements" establishes the category of bases and elements. This aggregate of consciousness, among the twelve bases, is called the mind base (manāyatana); among the eighteen elements, it is called the seven mind elements (citta-dhātu). Adding the mind element to the six consciousnesses, they are called the seven mind elements.