Assalamu alaykum!
I have an important question that is, there is a hadith where the prophet (Sal) said:
"WHEN A MAN MOUNTS ANOTHER MAN, THE THRONE OF ALLAH SHAKES"
I want to know whether this hadith is authentic and in which book of hadith is it mentioned with reference?
Jazakallah

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

There is no basis for this Hadeeth. It is a lie and it is fabricated, so it is not correct to attribute it to the Prophet .

As-Suyooti said in az-Ziyaadaat ‘ala al-Mawdhoo’aat:

“I found at the back of a copy of Ibn Abu Shaybah, with a Moroccan handwriting, and I do not know the name of its writer, what reads: “... Anas ibn Maalik said: "The Prophet said: “When a man mounts another man, the Throne (of Allah) shakes and the heavens would say: ‘O Lord! Let us stone him’, and the earth would say: ‘O Lord! Let us swallow him.’ Allah would say: “Leave him as his destination (fate) is to Me, and he will stand in front of Me.”

Others have written below this Hadeeth what reads: “Its chain of narrators is unfounded, and its wording is fabricated.”

Besides, Ash-Shawkaani said in al-Fawaa'id al-Majmoo’ah fi al-Ahadeeth al-Mawdhoo’ah:

“The Hadeeth “When a man mounts another man, the throne of Allah shakes and the heavens would say: ‘O Lord! Let us stone him’, and the earth would say: ‘O Lord! Let us swallow him.’, is fabricated.” [End of quote]

Allah knows best.