Parashas Acharei Mos · Passuk 16:26
Chabad.org — English translation + Rashi
Torah Temima points to Yoma 67b.
English: One might think impurity begins as soon as he leaves the azarah wall; the wording teaches that it begins only once he has left the wall of Yerushalayim.
Boundary of tumah onset
The garment-impurity of the sender takes effect from the moment he exits the walls of Yerushalayim, not earlier at the azarah and not only at the cliff.