Imam Sadiq narrated that Hazrat Mahdi (as) would appear in 1400 Hijri

Narrated from Imam Sadooq:

As for the disappearance of Isa (a.s.). The Jews and Christians formed unanimity that he has been killed; whereas Allah belied them in this verse: And they did not kill him nor did they crucify him, but it appeared to them so. Likewise is the disappearance of the Qaim since one group denies it for its length – ranging from ONE MISGUIDED PERSON WHO SAYS, “HE WAS NEVER BORN”; TO ANOTHER WHO DEVIATES BY SAYING, ‘THIS WILL INCREASE TO THIRTEEN AND UPWARDS’, and another who sins against Allah, the Mighty and Sublime by saying, ‘THE SPIRIT OF THE QAIM SPEAKS THROUGH THE BODY OF SOMEONE ELSE.

(Shaykh as-Sadooq, Kamal-Al-Deen, Volume 2, Chapter 33, Hadith 51)