פירוש על ביצה 30:5

What is the reason? – Why did (the Rabbis) ordain the eruv? The verse is certainly not speaking about the eruv, and the eruv is not heard from them, it is a mere support. When the verse comes, it teaches about saying Kiddush as it is taught in Masechet (tractate) Pesachim, remember Shabbat over wine. The eruv is from the Rabbis. What did they see to ordain this?

Rava said: select the choice portion – Since in the west, remember Shabbat and do not let Yom Tov consume everything. Choose a portion for this and a portion for that.