Holy Quran, Chapter 16 Verse 1

أَتَى أَمْرُ اللّهِ فَلاَ تَسْتَعْجِلُوهُ سُبْحَانَهُ وَتَعَالَى عَمَّا يُشْرِكُونَ

“The decree of Allah has come; therefore do not (seek to) hasten it. Glory be to Him, and far exalted is He above what they associate with Him”

Imam Sadiq (as) said, The first one who will pay allegiance to the Qaem (as) will be Jibraeel who will come down in the form of a white bird. After paying allegiance to the Qaem (as), he will place one of his legs on top of the Kaabah and the other one of top of Bayt Al-Muqaddas (Quds).

Then he will cry out loudly in an eloquent voice which will be heard clearly by everyone, “The decree of Allah has come; therefore do not (seek to) hasten it.” (Kamaluddin volume 2 page 671)

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Imam Sadiq (as) said, When Allah wills the rising of the Qaem (as),  He will send Jibraeel in the form of a white bird. Jibraeel will place one of his legs on top of the Kaabah and the other leg on top of Bayt Al-Muqaddas.

Then he will cry out loudly, “The decree of Allah has come; therefore do not (seek to) hasten it”.

Then the Qaem (as) will come to the Maqaam of Ibrahim and will perform two units of prayer. Thereafter, he and his three hundred and thirteen companions, who will surround him, will leave. Some of his companions will (be among those who travelled) at night from their beds and will (have met) the Qaem (as) by Hajar. Then the companions will disperse throughout the earth. (Dalael Al Imamah page 472)