Eruvin 64 a
Dr. Judah told me, he once asked his father in law (R. P. Hirshprung zt"l)
The gemara says If one wants to take leave from his friend, he should do
so by saying a dvar halacha, doing so, he will remember him.
He asked who is the one that is supposed to say the dvar halacha and who
remembers. Is it the one that stays on or the one who leaves.*
R.Hirshprung answered that probably the the one who is leaving says
the dvar halacha and the one who stays on, remembers.
Dr. Judah asked him maybe the opposite. R. Hirshprung answered yes
could be the opposite. (The one who stays on says the dvar halacha etc.)
D.J. asked again maybe both have to say the dvar halacha. R.S. answered
yes could be so.
D.J. said " I am surprised at the schver that he dosn't have an answer
from the gemara". R.S. asked him which seder in shas would he find
the answer. D.J. said that he wants to make it easier for the schver.
The answer is based on a gemara in eiruvin.
It didn't take more than a few seconds and Rabbi Hirshprung said
yes you are right the answer is in eiruvin 64a at the bottom of the amud.
* Seems from the Maharsha (Brachos 31a) the one who leaves says the dvar Halacha.