Reference for Bava Batra 127:2
א"ל רבינא לרב אשי תא שמע דאמר ריש לקיש זאת אומרת המוכר בית לחבירו ואמר לו על מנת שדיוטא העליונה שלי דיוטא העליונה שלו ואמרינן למאי הלכתא רב זביד אמר שאם רצה להוציא בה זיזין מוציא רב פפא אמר שאם רצה לבנות עלייה על גבה בונה
Rabina said to R. Ashi: Come and hear:<span class="x" onmousemove="('comment',' An argument against R. Dimi, from the ruling of R. Papa. ');"><sup>3</sup></span> Resh Lakish said: This shows that if a man sells an apartment to another with the stipulation that the top layer still belongs to him, the top layer does still belong to him; and we asked what was the purpose of the new rule laid down by Resh Lakish, and R. Zebid said: [In order to tell us] that if the vendor desires to let out projecting spars from the roof he may do so, and R. Papa said: [In order to tell us] that if he desires to build an upper chamber over the apartment he may do so. Now if you assume that the top layer is not transferred automatically, what does he gain by his stipulation?<span class="x" onmousemove="('comment',' Since even without this the vendor would still retain possession of the roof. ');"><sup>4</sup></span>