۱- “Sunan” of Abu Dāwood has 5274 ʼAḥādīth [Hadiths].

And twelve of them in the volume four, part “Kitāb al-Mahdi” are about Imam Mahdi (hgr).

۲-“Sunan” of Tirmidhi: In Abwāb al-Ghadr, there is a chapter named “Mā Jā’a fi al-Mahdi”.

In this chapter four ʼAḥādīth are explicitly [mentioned] about Imam Mahdi (hgr).

In addition to this chapter, there are a lot of Hadiths mentioned later [in this book], about The Descent of Jesus Christ, Seduction of Dajjāl [the Antichrist] and Apocalypse signs like: rising sun from morocco, etc.

“Sunan” of ibn Mājah: It’s another most authentic books of Sunni.

And in the part named “Kitāb al-Fitan” chapter “Khoruj al-Mahdi”, Seven narrations can be seen about Imam Mahdi (hgr).

The lifetime of all those three (Abu Dāwood, Tirmidhi, Ibn Mājah), was in the era of Imam Hasan Askari (pbuh) and early minor occultation.