Bava Batra 60B

זיל האידנא ותא למחר. בליליא, שדר קצייה לההוא דידיה.

Go now, and come tomorrow. At night, R' Yannai sent and had someone cut down that tree that belonged to him.

למחר אתא לקמיה, אמר ליה: זיל קוץ. אמר ליה: הא מר נמי אית ליה! אמר ליה: זיל חזי; אי קוץ דידי ⁠– קוץ דידך, אי לא קוץ דידי ⁠– לא תקוץ את.

The next day, that man came before R' Yannai, who said to him: Go, cut down your tree. The man said to him: But the Master also has a tree that leans into the public domain. R' Yannai said to him: Go and see: If mine is cut down, then cut yours down. If mine is not cut down, you do not have to cut yours down, either.

מעיקרא מאי סבר ולבסוף מאי סבר? מעיקרא סבר: ניחא להו לבני רשות הרבים, דיתבי בטוליה. כיון דחזא דקא מעכבי, שדר קצייה. ולימא ליה: זיל קוץ דידך, והדר אקוץ דידי! משום דריש לקיש, דאמר: ״התקוששו וקושו״⁠ – קשוט עצמך ואחר כך קשוט אחרים.

The Talmud asks: At the outset what did R' Yannai hold, and ultimately, what did he hold? The Talmud replies: At the outset, he held that the general public is amenable to having the tree there, as they sit in its shade. Once he saw that they were preventing someone else who owned a tree from keeping his, he understood that it was only out of respect that they did not object to his tree being there. He therefore sent someone to cut it down. The Talmud asks: But why did he tell the man to return the next day? Let him say to him: Go cut down your tree, and then I will cut mine down. The Talmud answers: Because of the statement of Reish Lakish, who said: The verse states: “Gather yourselves together and gather [hitkosheshu vakoshu]” (Zephaniah 2:1), and this can be explained homiletically to mean: Adorn [keshot] yourself and afterward adorn others, i.e., act properly before requiring others to do so.