Reference for Megillah 22:6

(אסתר א, ב) בימים ההם כשבת המלך וכתיב בתריה בשנת שלש למלכו אמר רבא מאי כשבת לאחר שנתיישבה דעתו אמר בלשצר חשב וטעה אנא חשיבנא ולא טעינא

(Esther 1:2) "In those days when the king sat..." And it is written afterward: "Three [years into] his reign..." Rava said: What is [the meaning of] 'when [the king] sat'? That after [some time on the throne] his mind was rested. [Ahashverosh] said: Belshazar calculated, but was mistaken. I calculated and am not mistaken.

Jastrow