佛說觀佛三昧海經 4.39

4.39

The Buddha told Ānanda: “After the Buddha’s parinirvāṇa, if the Buddha’s disciples wish to see this mark, they should give rise to a benevolent mind and cultivate the precept of non-killing universally for the sake of all sentient beings in the ten directions. Those who perform this practice, even if they do not sit in meditation, will constantly be able to see the diverse colors in their dreams, just as described above.”