大乘起信論廣釋卷第三．四．五 5.57

5.57

論。云何為五至止觀門　釋曰。此依數列名也。以諸菩薩定惠雙修止觀齊運二不相離故。合二度為第五門。

The treatise says: "What are the five? … the gate of calming and insight." The commentary says: This enumerates the names according to the number. Because bodhisattvas cultivate concentration and wisdom in tandem, exercising calming and insight simultaneously without the two separating from each other, it combines the two perfections into the fifth gate.