俱舍論記 8.50

8.50

「當言傍住至不說上下」者，說一切有部答。說有八方世界傍住。經中既說四方，不說上、下，明知傍布。經說四方，攝四維也。復言雨滴無間無斷，此喻世界無邊無間無斷，或壞或成無暫時息。

"It should be said that they abide horizontally [...] without speaking of above and below": the Sarvāstivāda replies. They state that the worlds in the eight directions abide horizontally. Since the sūtra speaks of the four directions and does not speak of above and below, it is clear that they are arranged horizontally. When the sūtra speaks of the four directions, it subsumes the four intermediate directions as well. Furthermore, it says that the raindrops fall without interval and without interruption; this is an analogy for the worlds being boundless, without interval, and without interruption, whether dissolving or forming without ceasing for an instant.