俱舍論頌疏論本 2.54

2.54

釋曰：內有十二，六根、六識。此十二界心所依故，所依親近，說名為內。色等六境為心所緣，所緣疎遠，說名為外。

Explanation: Twelve are internal: the six faculties and the six consciousnesses. Because these twelve elements are the bases of mind, and the bases are close and intimate, they are said to be 'internal'. The six objects—visible form and the rest—are the objective supports of mind; and because the objective supports are distant and mediated, they are said to be 'external'.