Abu Dawud said: Ibn Ishaq and al-Awza'i narrated it from Yahya ibn Sa'id, similar to it. (1)
And Malik narrated it from Yahya ibn Sa'id, who said: He observed i'tikaf for twenty [days] of Shawwal. (2)
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= Al-Bukhari also recorded it (2033), (2034), and (2041), Muslim (1173), and al-Nasa'i (3333) via multiple chains from Yahya ibn Sa'id, with it.
It is in "Musnad Ahmad" (24544), and "Sahih Ibn Hibban" (3666).
Al-Khattabi said: In it there is jurisprudence that the one observing i'tikaf begins his i'tikaf at the beginning of the day, and enters his place of i'tikaf after praying the dawn prayer. Al-Awza'i held this view, and Abu Thawr said likewise.
Malik, al-Shafi'i, and Ahmad said: He enters i'tikaf before sunset if he intends to observe i'tikaf for a specific month, and this is the school of the people of opinion (ahl al-ra'y).
(1) Al-Bukhari recorded it (2045), Muslim (1173), and al-Nasa'i in "al-Kubra" (3331) via the chain of al-Awza'i, and Muslim (1173) via the chain of Muhammad ibn Ishaq, both from Yahya ibn Sa'id, with it.
(2) Thus it occurs in the sources: twenty. Ibn Ruslan said: The preserved wording is "ten of Shawwal." Ibn Hibban recorded it in his "Sahih" (3667) via the chain of 'Amr ibn al-Harith, from Yahya ibn Sa'id, with it, with the wording: "Then he observed i'tikaf in twenty of Shawwal," and this is the wording found in "al-Taqasim wa al-Anwa'." The wording of the author of "al-Ihsan fi Taqrib Sahih Ibn Hibban" is: "He observed i'tikaf in ten of Shawwal," and this is correct.
The author of "Bazl al-Majhud" 11/350 said, commenting on his statement "he observed i'tikaf for twenty of Shawwal": This statement contradicts what al-Bukhari recorded in his "Sahih" (2034) from Malik, from Yahya ibn Sa'id, from 'Amrah, from 'A'ishah, in which it says: "He did not observe i'tikaf until he observed i'tikaf for ten of Shawwal."
And more than one of the imams narrated it via chains from Yahya ibn Sa'id, with it, and these are cited in our commentary on "al-Musnad" (24544).