From the 《
妙法莲华经·
方便品》 (Lotus Sutra):
如优昙钵花,
时一现耳 — 'like the udumbara flower, which appears only once in an age.' In Buddhist lore, the udumbara blooms once every 3,000 years;
昙花 in modern Chinese refers to the night-blooming cereus, which opens for only a few hours after dark.