俱舍論疏 17.59

17.59

「論曰至從一唯至八」，此總舉不善業道從一唯至八，不至九、十，不善業道不並起故。既言與思俱轉，定是思業道，同剎那義，不容異解。

The commentary says: "From one up to eight only": this generally points out that unwholesome karmapathas operate only from one up to eight, not reaching nine or ten, because unwholesome karmapathas do not arise concurrently. Since it says "operate together with intention", it definitely refers to intentional karmapathas operating in the same instant; this admits of no other interpretation.