Jami` at-Tirmidhi 803
جامع الترمذي#803SahihAnas bin Malik narrated:"The Prophet would perform I'tikaf during the last ten (nights) of Ramadan. One year he did not perform I'tikaf, so he performed I'tikaf for twenty (nights) in the following year
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Ahmad Muhammad ShakirSahih
Al-AlbaniSahih
Bashar Awad MaaroufHasan Sahih
Zubair Ali ZaiSahih
In-book reference: 8:122