俱舍論頌疏論本 15.93

15.93

釋曰：於佛上首僧者，佛於僧中最為上首，名上首僧。佛雖非聲聞僧，而是聖僧攝也。

The explanation says: Regarding "Toward the Buddha, the foremost saṃgha": The Buddha is the foremost within the Saṃgha, hence called the foremost saṃgha. Although the Buddha is not a saṃgha of śrāvakas, he is subsumed under the saṃgha of noble ones.