The voice from sky (Arabic: صوت من السماء), according to Shia Islamic eschatology, will be words spoken by Jibrael which is one of the signs of Mahdi's appearance.
[1][2][3] Though the voice from sky is claimed as a convinced sign but there is a discrepancy in the event time.
In compliance with Al-Shaykh Al-Mufid, as well as others, there is a Hadith which state a synchronicity between reappearance of Muhammad al-Mahdi and the voice from sky.
Based on another narration it will happen on Thursday night, twenty-third day of Ramadan.
[4][5][6] As claimed by most narration, contents of the voice will be about Muhammad al-Mahdi and his characteristics; and it will invite people towards him.