The Mahayana Mahaparinirvana Sutra 大般涅槃经

The Mahāyāna Mahāparinirvāṇa Sūtra （大般涅槃经）or Nirvana Sutra is an essential Mahayana Buddhist sutra which the Buddha discusses in length about Buddha-nature. It is the last teachings givenby the Buddha before he entered Parinirvana (in fact, the Buddha never really entered Parinirvana, he merely disappeared for the beings with lesser good roots and to teach people about impermanence). 

The "nature of the Buddha" is presented as a timeless, eternal "Self" (which is also non-self), the tathagatagarbha, the innate possibility in every sentient being to attain Buddhahood and manifest this timeless Buddha-nature. "It is obvious that the Mahaparinirvana Sutra does not consider it impossible for a Buddhist to affirm an atman provided it is clear what the correct understanding of this concept is, and indeed the sutra sees certain advantages in doing so."

The Buddha also explicitly prohibits his disciples from eating meat of any kind in this Sutra. The main reason for the prohibition is because eating meat kills the seed of great compassion in one who consumes it.

One can download an English translation here and a Chinese translation here.

Note - There are many versions of the Mahaparinirvana Sutra. The version which was translated by Faxian with only 6 volumes describes the events before the Buddha’s Mahaparinirvana and how Ananda was under the influence Mara and as a result did not invite the Buddha to stay longer in the world despite the Buddha’s repetitive hints.