出處：後漢書，公孫穆傳。
釋義：舊社會裡貧富差距很大，不但婚姻必須門當戶對，即使男人之間交朋友也必須講求身份。富豪吳裕願意紆尊降貴和窮人公孫穆做朋友是極為罕見的例子，所以後人就根據此一故事衍生一個成語“杵臼之交”。凡富人與窮人做朋友時即可說“杵臼之交”。
故事：後漢時代，有位當事人名公孫穆，十分好學。當他讀書完成一個階段後，很想深造，進大學研究學問，可是想進大學讀書需要很多的錢，而公孫穆家裡很窮，拿不出這筆巨額學費，進大學發生困難。正焦急時，突然聽到本區有位富豪名吳裕，正在招募一批舂米工人。公孫穆一想，不如先去做工，賺點錢作學費，將來再去上大學也不遲。他裝成一個工人模樣，前往應徵。錄取後就和其他工人在舂米場作舂米工作。一天，吳裕到舂米場巡查，看到公孫穆這個工人，舉止斯文，不像是個工人；問其原因，原來是個飽學之士。那時候工人都是文盲。吳裕對公孫穆的苦學精神十分佩服，便不顧貧富懸殊，就在舂米的杵臼前，交起朋友來。在當時的環境，貧富階觀念很深，吳裕肯紆尊降貴，願意跟一個窮讀書人做朋友是萬分難得的了。由於吳裕和公孫穆交朋友傳開來成為一段佳話，後人將此故事衍生為一個成語 “杵臼之交”。
用法：河南虞城橋集大寨的楊善人是個大地主，是橋集首富，但他夫婦因為節儉過度，中年時雙方變成瞎子。1936年他聘請家父在他家當師爺（老師），開館授徒。我家極貧，他卻與家父是很好的朋友，他們可說是“杵臼之交”。不幸，民國26（1937）年，家鄉淪陷，橋集大寨被日本兵燒光，楊善人家也不能倖免，所以家父就失業了，開始從事於抗日游擊隊工作，直至抗戰勝利。
One day, Wu Yu patrolled his rice-pounding workshop and found that there was a worker who was quite different with other workers, he seemed not like a coolie, and his manners were elegant and looked like a gentleman, so Wu Yu asked the worker’s details and knew that Gong Sun Mu was an erudite scholar. Wu Yu respected and admired Gong Sun Mu’s diligence in studying. Then, the two men—a boss and a coolie (the boss’ one employer) had a honest and hearty chatting and after the chatting, Wu Yu moved to make friends with Gong Sun Mu rice-pounding workshop before the mortar and pestle. This story had been disseminated as a favorite tale widely and long. In ancient times, wealthy class and poor class were separated clearly and would not make friends with each other. It’s a very special tale that Wu Yu would like to make friends with such a poor literate Gong Sun Mu. Since then, that a rich man makes friends with a poor man is called “friendship made before the mortar and pestle.”