俱舍論疏 6.163

6.163

論「有時四蘊至及彼生等」，明四蘊為因也。於無色界唯有二類，於三界中無有三蘊為因共感一果，有緣極少必有四蘊，無緣極多唯二故。

On the Śāstra's: "Sometimes four skandhas [...] and their arising, and so forth": this clarifies four skandhas serving as cause. In the formless realm there are only two categories. Across the three realms, there are never three skandhas serving as cause to jointly bring about a single result: where there is an objective support, at a minimum there must be four skandhas; where there is no objective support, at a maximum there are only two.