俱舍論記 13.210

13.210

「有餘部說至那伽臥在定」者，敘大眾部等計。佛常在定，心唯是善，無無記心，經說如來四威儀中常在定故。那伽，此云龍，顯世尊也。

"Other schools say... the Nāga is in concentration even while lying down": this relates the position of the Mahāsāṃghika and others. The Buddha is constantly in concentration, and his mind is solely wholesome without any indeterminate minds, because the sūtras say that the Tathāgata is constantly in concentration across the four postures. Nāga here means 'dragon', signifying the World-Honoured One.