俱舍論頌疏論本 14.115

14.115

釋曰：學處者，戒名學處，所應學故。越者，違越。若開誑語，便越學處，被檢問時，於所犯戒便言我不作。因斯於戒多所違越，故佛為欲令彼堅持，於一切律儀皆制虛誑語。

The explanation says: "Training rules" refers to the precepts, which are called training rules because they are what ought to be trained in. "Transgress" means to violate. If false speech were permitted, one would transgress the training rules: when questioned and investigated, regarding a precept one had broken, one would simply say, 'I did not do it.' Because of this, one would commit many violations of the precepts. Therefore, wishing to make them uphold them firmly, the Buddha prohibited false speech in all disciplines.