Hadith-

In the Name of Allah, the Most Gracious, the Most Merciful.

Ibn 'Omar said, "The Messenger of Allah, may Allah bless him and grant him peace, used to do i'tikaf for the last ten days of Ramadan." [Agreed upon]

'Aisha reported that the Prophet, may Allah bless him and grant him peace, used to do i'tikaf for the last ten days of Ramadan until Allah Almighty caused him to die. Then his wives did i'tikaf after him. [Agreed upon]

Riyadh

as-Salihin (The Meadows of the Righteous) of Imam Mohieddine Annawawi 631-676. Chapter 222:"Book of I'tikaf  ",Hadiths No. 1268 and 1269.