The Pratyutpanna Samādhi Sutra

The Pratyutpanna Samādhi Sutra derives its name from the pratyutpannabuddhasaṃmukhāvasthita-samādhi, the spiritual state in which by mental concentration one is able to see buddhas appearing before one’s very eyes. This sūtra describes the techniques involved  and gives Amitāyus, who resides in the western paradise of Sukhāvatī, as an example of a buddha who might appear in such a manner.

One of the oldest Mahāyāna sūtras, this is the earliest reference to Amitāyus among all the sūtras. It can thus be regarded as a forerunner of the Pure Land Sūtras.

Source
Skt. Pratyutpannabuddhasaṃmukhāvasthita-samādhi-sūtra. Translated into the Chinese by Lokakṣema as Banzhou sanmei jing (般舟三昧經). 3 Fascicles.

The Śūraṅgama Samādhi Sutra

The Śūraṅgama Samādhi Sutra expounds the essentials of meditative practice. When asked by the bodhisattva Dṛḍhamati, the Buddha replies that the śūraṅgama-samādhi(“Samādhi of the Heroic March”) is foremost among all methods of spiritual training, embracing within it all other methods of practice, and he then goes on to describe it in detail, explaining its powers and how to go about practicing this method. Viewed historically, the thought presented in this sūtra anticipates such works as the Avataṃsaka-sūtra, the Vimalakīrtinirdeśa-sūtra, and the Saddharmapuṇḍarīka-sūtra, and it is considered to have been composed around the start of the Common Era.

Source
Skt. Śūraṅgamasamādhi-sūtra. Translated into the Chinese by Kumārajīva as Shoulengyan sanmei jing (首楞嚴三昧經). 2 fascicles.