He and a companion were forced to pull a wheel of fire on which a wrathful hell-guard was sitting, holding a burning club with which to beat them. His companion was so weak that he couldn’t pull the wheel anymore. The hell-guard stabbed his companion with a burning trident. His companion kept crying loudly and bleeding profusely. At that moment, with strong love and compassion, the Buddha developed enlightened aspiration, a vow to take responsibility for helping his companion and all the suffering beings from the depth of his heart, and he became a bodhisattva for the first time.
The bodhisattva begged the hell-guard, “Please have a little mercy on my suffering companion.” At that, in a rage the hell-guard hit him with a burning trident. Because of the power of his strong compassion, the bodhisattva died and was liberated from the hell-realm. His evil deeds of many eons were purified instantly by the power of such enlightened aspiration. Thereafter, he started his journey toward the fully enlightened state of buddhahood.
From Incarnation: The History and Mysticism of the Tulku Tradition of Tibet by Tulku Thondup, pages 24–25