I've run across variations of the following:
קיימא לן דהלכתא כרבא בכל מקום פרט ליע"ל קג"ם
That the halacha is like Raba in every place in the Talmud except six place where it is like Abaye.
The most I've come across so far is that this is a principle that the Rambam held by, but it seems to me like there must be an earlier source to this.
What is the source of this and what are the six places whose initials are given above?
As this phrase does appear in our Gemaras, almost all of the Rishonim assume it to be true. The acronym stands for: (this explanation appears in Rashi to Bava Metzia 22b ( בבא מציעה פרק ב דף כב,ב ) and Kidushin 52a ( קידושין פרק ב דף נב,א ), where Tosfos argues there)
The actual phrase is:
תיובתא דרבא תיובתא והלכתא כוותיה דאביי ביע"ל קג"ם
[This is] a complete refutation of Rava. And the law is in accordance with Abaye in [the cases indicated by the initials] Y'AL KGM.
יאוש שלא מדעת - הלכה כאביי שלא הוי יאוש
"Yeush shelo midaas" - if a person loses something without realizing it, but once they do realize it, will give up looking for it and deem it owner-less, than a person who finds it can keep it (Bava Metzia 21b)
עד זומם - הלכה כאביי שנפסל למפרע
"Ed Zomem Nifasl Lemafrea" - if two witnesses are deemed Eidim Zomemim (conspiring witnesses), then even their testimony that they've given in the past in nullified. (Sanhedrin 27a and Bava Kamma 74b)
According to Rashi, the ל stands for
לחי העומד מאליו - הלכה כאביי שהוי לחי
"Lechi Ha'omeid Me'eilav" - if a stick is standing by itself, it can still count as a stick for purposes of dividing a mavoy from a public domain (Eiruvin 15a)
According to Rabbeinu Tam, ל stands for
לידה - הלכה כאביי שאינה ימי זיבה
Leida - birth; the law is like Abaye in that a woman who gives birth during the days of zivah doesn't interrupt her count and can count clean days within the post-birth pure period
קידושין שלא נמסרו לביאה - הלכה כאביי שהוו קידושין
"Kidushin Shelo Nimseru Le'biah" - if a man attempts to marry a woman, but his act of marriage cannot lead to permissible intimacy (i.e. he attempted to marry a woman and her sister), then it still counts as kiddushin (i.e. requires a divorce) (Kiddushin 52b)
גילוי דעת בגיטין - הלכה כאביי שלא הוי מילתא
"Giluy Daas" for divorce - if a man can reveal that he wanted a stipulation regarding a divorce document that he had already sent to a messenger to give to his wife (Gittin 34a)
מומר אוכל נבילות להכעיס - הלכה כאביי שפסול ולא צריך דווקא פסול ד"חמס"
"Mumar Ochel Neveilos" - a rebel who eats forbidden foods with the intent to anger God is not acceptable as a court witness (Sanhedrin 27a)
