大乘起信論義記 3.446

3.446

Furthermore, relying on this samādhi, one knows that the dharmadhātu has a single mark, which means that the dharmakāya of all Buddhas and the bodies of sentient beings are equal and non-dual; this is called the single-practice samādhi. One should know that tathatā is the root of samādhi; if a person cultivates it, they will gradually be able to produce immeasurable samādhis.