The Mahāyāna Great Assembly Sūtra of the Ten Wheels of Kṣitigarbha, Fascicle Six: The Chapter on the Practice of Having Reliance

Original text:善男子，是故若欲于三乘中，随依一乘求出生死，欣乐安乐厌危苦者，应于如来所说正法，或依声闻乘所说正法，或依独觉乘所说正法，或依大乘所说正法，普深信敬，勿生谤毁、障蔽、隐没下至一颂，常应恭敬、读诵、听闻，应发坚牢正愿求证。

谤毁三乘随一法者，不应共住下至一宿，不应亲近咨禀听法。若诸有情，随于三乘毁谤一乘，或复亲近谤三乘人咨禀听受，由此因缘，皆定当堕无间地狱，受大苦恼，难有出期。

何以故？善男子，我于过去修菩萨行，精勤求证无上智时，或为求请依声闻乘，所说正法下至一颂，乃至弃舍自身手足、血肉皮骨、头目髓脑；或为求请依独觉乘，所说正法下至一颂，乃至弃舍自身手足、血肉皮骨、头目髓脑；或为求请依于大乘，所说正法下至一颂，乃至弃舍自身手足、血肉皮骨、头目髓脑。如是勤苦，于三乘中下至求得一颂法已，深生欢喜，恭敬受持，如说修行，时无暂废，经无量劫修行一切难行、苦行，乃证究竟无上智果；复为利益安乐有情，宣说开示三乘正法。以是义故，不应谤毁、障蔽、隐没下至一颂，常应恭敬、读诵、听闻，应发坚牢正愿求证。

善男子，如是三乘出要正法，一切过去、未来、现在过殑伽沙诸佛同说，大威神力共所护持，为欲拔济一切有情生死大苦，为欲绍隆三宝种姓令不断绝。是故于此三乘正法，应普信敬，勿生谤毁、障蔽、隐没。若有谤毁、障蔽、隐没三乘正法，下至一颂，决定当堕无间地狱。

Explanation: The Buddhist scriptures say that those who dwell together with someone who slanders any one of the vehicles among the three vehicles will also fall into the Avīci Hell. How much more so for those who spread speech slandering the Mahāyāna Dharma — spreading Theravāda Buddhism, spreading the doctrinal teachings of Theravāda masters — is tantamount to destroying the Buddha's teachings, plunging sentient beings into the ocean of suffering, causing them to remain submerged for endless kalpas. Therefore, such karmic obstructions are extremely profound and heavy, and liberation is hard to attain.