摩訶止觀 7.190

7.190

行緣既爾，住、坐、臥、語、作，例前可解。三三昧無臥法，隨自意則有。昔國王於臥中悟辟支佛，當知臥中得有觀行（云云）。

Since the condition of walking is like this, standing, sitting, lying down, speaking, and physical activity can be understood by analogy with the previous. The three samādhis have no practice of lying down, but it is present in [the samādhi of] following one's own thought. In the past, a king awakened as a pratyekabuddha while lying down; one should know that contemplative practice can take place while lying down (and so forth).