The Mahayana Mahaparinirvana Sutra 大般涅槃经

When the Buddha “entered Parinirvana” 佛陀“入般涅槃”

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 by the Buddha before he entered Parinirvana (in fact, the Buddha never really entered Parinirvana, he merely disappeared for the beings with less roots and to warn 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."

One can download a pdf copy here.