Eruvin 88B

פחות מארבע ⁠— שופכן. אי דעביד עוקה ⁠— שרי, אי לא ⁠— אסור.

But if the courtyard is less than 4 cubits by 4 cubits in area, one simply pours the water out, as the place is not fit for sprinkling. Therefore, if one fashioned a pit, it is permitted to pour out water; but if not, it is prohibited to do so, as one certainly intends for the water to flow outside.

רבי זירא אמר: ארבע אמות ⁠— תיימי, פחות מארבע אמות ⁠— לא תיימי.

R' Zeira offered a different reason and said: In a courtyard of 4 cubits by 4 cubits, the water is likely to be absorbed into the ground. If it is less than 4 cubits in size, the water will not be absorbed but will flow out.

מאי בינייהו? אמר אביי: אריך וקטין איכא בינייהו.

The Talmud asks: What is the practical difference between these two explanations? Abaye said: There is a difference between them with regard to a long and narrow courtyard. As the area of this courtyard is also 16 square cubits, it likewise absorbs the water. R' Zeira would therefore rule that it does not require a pit. However, as this courtyard is not in need of sprinkling, it requires a pit according to Rabba.

תנן: חצר ואכסדרה מצטרפין לארבע אמות. בשלמא לרבי זירא ניחא, אלא לרבה קשיא!

We learned in the Mishnah: A courtyard and a portico combine for the requisite 4 cubits, permitted the pouring of water into a courtyard that lacks a pit. The Talmud asks: Granted, according to the opinion of R' Zeira, this works out well, as the total area is large enough to absorb the water. However, according to Rabba it is difficult, for when the courtyard is joined with the portico it is no longer in the shape of a square, and it is therefore unfit for sprinkling.