Many Buddhists think that the practices and teachings of the Mahayana, Mantrayana, and Vajrayana were invented several centuries after Shakyamuni established the Dharma. In fact, almost all developments can be shown to have their origin in Shakyamuni's teachings as found in the early sutras (Pali and Agama). Examples include such essential Mahayana concepts as shunnyata, vajra, vajra-holding dieties, and Dharmakaya.
The Dharmakaya Buddha meditation is a cornerstone of Mahayana, one of the 'Three Bodies of the Buddha'. Here is the story where Shakyamuni describes the Dharmakaya, paraphrased from the English translation by Bhikkhu Bodhi, and exactly as would be described by Mahayana practitioners.
One of the great elder disciples, Vakkali, whom the Buddha called 'Foremost in Faith', wanted to see the Buddha one more time before his own death, but while traveling he reached a place and could go no farther. The Buddha hearing this went to see Vakkali and then asked him if he had any remorse or regret, to which Vakkali replied, "Quite a lot." Further inquiry reveled the Vakkali had led a pure life in regard to morality, and so the Buddha asked, "Then why do you have remorse and regret?" Vakkali explained that it was because he was unable to travel to see the Buddha. At this point in the story, the Buddha explains the Dharmakaya:
"Enough Vakkali! Why do you want to see this foul body? For in seeing the Dharma, Vakkali, one sees me; and in seeing me, one sees the Dharma."
This is the reason why today one is able to directly see the Buddha, even 2500 years after his earthly existence. In meditation, the syllable AH symbolizes the emptiness and equality of both the Buddha and the Dharma.