פירוש על מלכים א 3:15

Shlomo woke up and behold it was a dream. And behold he understood that his dream was true. He heard a bird chirping and understood its language, a dog barking and he understood its language.

And [he] made a feast. Out of his heart’s happiness [he celebrated] because he realized that his dream was true.²⁴Shlomo celebrated his newly acquired wisdom. Rabbi Elazar said that from here we derive the custom of making a festive meal [e.g., siyum] to honor the completion of reading the Torah or studying a Masechta. See Shir Hashirim Rabboh 1:9.