zhēnjiè
仰針於地，自天上投芥子，欲適中其針鋒甚難。以譬佛出世之難遇也。南本涅槃經純陀品曰：「芥子投針鋒，佛出難於是。」天名疏曰：「仰針於地，梵宮投芥，墮在針鋒。此事甚難，值佛生信復難於是，生信聞法復難於是。」止觀五曰：「墜芥豈得下貫針鋒。」
Needle and mustard seed; the appearance of Buddha is as rare as hitting the point of a needle on earth by a mustard seed thrown from the sky.
a needle and a small particle [attract each other]
一針芥 又作針鋒。仰一針於地，自天上投一芥子，欲適巧投中針鋒甚難，以此比喻值佛出世之難。〔南本涅槃經純陀品、摩訶止觀卷五〕p5477