Hadith.4402 - It is narrated from Jabir ibn Abdullah Al-Ansari that he said:
When the Messenger of Allah سبحانه وتعالى, peace be upon him and his family, married Sayyidah Fatimah {s.a} to Imam Ali ibn Abi Talib عليه السلام, peace be upon them, some people from Quraysh came to Him صلى الله عليه وعلى آله وسلم and said: "You have married Ali عليه السلام with a trivial dowry."
The Prophet, peace be upon him and his family, replied to them:
"It was not I, who married Ali عليه السلام, but Allah سبحانه وتعالى, Mighty and Majestic, married Him عليه السلام to Her {s.a}.
On the night I was taken on the Night Journey to the Sidrat al-Muntaha (Lote Tree at the Utmost Boundary), Allah سبحانه وتعالى, the Mighty and Majestic, revealed to the Lote Tree, 'Scatter your ornaments.' It then scattered pearls and jewels upon the maidens of Paradise, who exchanged them as gifts and took pride in them, saying, 'This is from the scattering for Sayyidah Fatimah {s.a}, the daughter of Muhammad صلى الله عليه وعلى آله وسلم, peace be upon him and his family.'"
When the night of the wedding arrived, the Prophet, peace be upon him and his family, brought his grey mule and covered it with a cloak. He صلى الله عليه وعلى آله وسلم said to Sayyidah Fatimah {s.a}, peace be upon her, "Mount it."
He صلى الله عليه وعلى آله وسلم then commanded Salman, may Allah سبحانه وتعالى have mercy on him, to lead it, while the Prophet, peace be upon him and his family, walked behind it.
While they were on the way, the Prophet, peace be upon him and his family, heard a sound and saw Archangel Jibril (Gabriel) عليه السلام, descending with seventy thousand angels, and Mika'il (Michael), peace be upon him, with seventy thousand angels.
The Prophet, peace be upon him and his family, said: "What has brought you down to earth?"
They said: "We have come to escort Sayyida Fatimah {s.a}, peace be upon her, to her husband."
Archangel Jibril عليه السلام proclaimed "Allahu Akbar" (Allah سبحانه وتعالى is the Greatest), and Mika'il, peace be upon him, followed with the same proclamation, as did the angels.
Then Muhammad صلى الله عليه وعلى آله وسلم, peace be upon him and his family, also proclaimed "Allahu Akbar." From that night, the custom of proclaiming "Allahu Akbar" during weddings was established.