Rif Pesachim 26b {Pesachim 118a continues ... 119b}

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26b

{Pesachim 118a continues}
The Sages learnt {in a brayta}: The fifth, he says upon the Great Hallel. These are the words of Rabbi Tarfon. Others say {that he should say upon it Tehillim perek 23} "The Lord is my Shepherd; I shall not want."

From where does the Great Hallel begin?
Rav Yehuda said: From {Tehillim perek 136} Hodu

{that is, the perek that begins:

א הוֹדוּ לַה' כִּי-טוֹב: כִּי לְעוֹלָם חַסְדּוֹ.	1 O give thanks unto the LORD, for He is good, for His mercy endureth for ever.
ב הוֹדוּ, לֵאלֹקֵי הָאֱלֹהִים: כִּי לְעוֹלָם חַסְדּוֹ.	2 O give thanks unto the God of gods, for His mercy endureth for ever.

}
until {the next perek, Tehillim 137} עַל נַהֲרוֹת בָּבֶל.

And Rabbi Yochanan said: From {Tehillim perek 120} "Shir HaMaalot" until עַל נַהֲרוֹת בָּבֶל.

Rav Yaakov bar Acha said: From כִּי-יַעֲקֹב, בָּחַר לוֹ יָהּ {Tehillim perek 135:4, but presumably from the beginning of the perek and the reference is just to uniquely identify the perek} until עַל נַהֲרוֹת .בָּבֶל

And the practice in the world is like Rav Yehuda in both of them - in regard to the Blessing of Song and the Great Hallel.

{Pesachim 119b}
Mishna:
We are not maftirin after the pesach with afikoman.
{Soncino renders: One may not conclude after the paschal meal [by saying]: 'Now to the entertainment [afikoman].'}