俱舍論疏 27.101

27.101

論「傳說此四至無礙解故」，述《婆沙》一說也。此四加行成，方得四無礙故。

The Śāstra states: "It is traditionally said that these four up to because of unobstructed understanding," recounting one explanation from the Vibhāṣā. It is because when these four preparatory applications are completed, one only then attains the four unobstructed understandings.