The Prophet (ﷺ) used to spend the last ten days of Ramadan in I’tikaf. One year he was traveling, so the following year he spent twenty days in I’tikaf.
Sunan Ibn Majah Book 7, Hadith 133
← Previous Hadith “The Prophet (ﷺ) used to observe I’tikaf for ten days every year. In the year in which he passed away, he observed I’tikaf for twenty days. And the Qur’an would be reviewed with him once every year, but in the year in which he passed away, it was reviewed with him twice.”

Sunan Ibn Majah Book 7, Hadith 132 Next Hadith → “When the Prophet (ﷺ) wanted to start I’tikaf, he would pray the Subh, then he would enter the place where he wanted to observe I’tikaf. He wanted to spend the last ten days of Ramadan in I’tikaf, so he ordered that a tent be set up for him.” Then ‘Aishah ordered that a tent be set up for her, and Hafash ordered that a tent be set up for her. When Zainab saw their two tents, she also ordered that a tent be set up for her. When the Messenger of Allah (ﷺ) saw that, he said: “It is righteousness that you seek?” Then he did not observe I’tikaf during Ramadan, and he observed I’tikaf during ten days of Shawwal.

Sunan Ibn Majah Book 7, Hadith 134