Reference for Bava Batra 44:1
הבא לסמוך לא יסמוך אלא אם כן הרחיק ממנו ארבע אמות מתקיף לה רבא והא מי שהיה כותלו סמוך לכותל חבירו קתני
If a man wants to build a wall alongside of his neighbour's wall, he must not do so unless he keeps it [at least] four cubits away. Raba strongly objected to this, on the ground that it says. IF A MAN [ALREADY] HAS A WALL RUNNING ALONGSIDE OF HIS NEIGHBOUR'S WALL. No, said Raba: what it means is this: If a man had a wall running alongside of his neighbour's wall at a distance of four cubits and it falls down, he must not bring another wall alongside unless he keeps it four cubits away,<span class="x" onmousemove="('comment',' Tosaf. points out that this would imply that according to Raba the second wall must be four cubits away only if the first was also, which is incorrect. ');"><sup>1</sup></span>