俱舍論疏 14.152

14.152

論「是故近事必具律儀」，結有部也。若別說戒相時得，容有不能持者不發戒。既總受三歸自總稱我是近事即發戒故，支必具五。

The commentary states: "Therefore, an upāsaka necessarily possesses the complete saṃvara", concluding the Sarvāstivāda view. If they were obtained when the characteristics of the precepts are spoken separately, there might be cases where those incapable of upholding them would not engender the precepts. Since collectively receiving the three refuges and personally declaring collectively 'I am an upāsaka' engenders the precepts, the factors must completely number five.