佛說觀佛三昧海經 7.34

7.34

'Then in the sky there was an exquisite sound praising the four immeasurables, and after that distinguishing the realm of emptiness and non-being, the realm of quiescence devoid of thought and mindfulness. One who performs this contemplation is called one who contemplates the Tathāgata reclining. One who contemplates the Tathāgata reclining should first contemplate a reclining image. Having seen the reclining image, one should form this thought: "When the Buddha was in the world, the reason he manifested reclining was that the bodies of the Buddhas, the Tathāgatas, are without fatigue; it was solely to subdue the obstinate and powerful mallas and beings of evil views and unwholesomeness, or out of compassion for the monks, that he manifested reclining on his right flank." The Tathāgata's reclining is the reclining of great compassion. If one wishes to contemplate the Buddha reclining, one should practice the mind of benevolence.
Practicing the mind of benevolence means: taking all sentient beings as an object, when seeing them suffer, one does not spare one's own body to bring about their peace and joy, causing suffering beings to be free from misfortune. The mind of great compassion means: when seeing sentient beings suffering affliction, regarding them as one's own parents, teachers, and good friends, giving rise to a grieving mind with tears like a heavy rain. Such minds are called great compassion. When seeing others receive joy, joy arises in the mind, just as when a monk attains the third dhyāna; this is called joy.