Knowledge of the unseen is extremely vast, and Allāh, The All Knowledgeable, may occasionally choose to reveal some of it to some of His Prophets and Messengers who may then relay it to us. Allāh said about Himself:

عَالِمُ الْغَيْبِ فَلَا يُظْهِرُ عَلَىٰ غَيْبِهِ أَحَدًا ‎﴿٢٦﴾‏ إِلَّا مَنِ ارْتَضَىٰ مِن رَّسُولٍ

“He is the Knower of the unseen. He only discloses it to selected Messengers.”
[al-Jinn, 72:26-27]

Believing in any revealed knowledge of the unseen then becomes a part of faith.

Saʿd ibn Muʿādh (رضي الله عنه) was a revered Companion. He was from the Anṣār as he was from the Aws tribe resident in Madīnah. In fact, he was a chief. Consequently, his conversion led to the immediate conversion of his entire subtribe. He fought in many battles alongside the Messenger (صلى الله عليه وسلم). A miraculous event occurred when he died. The Prophet (صلى الله عليه وسلم) said: “The throne of The Most Merciful shook when Saʿd ibn Muʿādh died.” (Reported by al-Bukārī (2634) and Muslim (2466))

Al-Nawaī (رحمه الله) said: “Happiness caused the throne to shake because it knew that Saʿd’s soul was approaching.” (Ṣharḥ Ṣaḥīḥ Muslim, vol. 16, pg 22)

Saʿd ibn Muʿādh (رضي الله عنه) died a martyr, since he died shortly after the Battle of the Trench from an injury he sustained, and the Prophet (صلى الله عليه وسلم) explained that the martyrs’ souls reside in the bodies of green birds nestling in chandeliers/lanterns suspended from the Almighty’s throne. So the throne was happy because his soul was coming to nestle in one of these chandeliers/lanterns.

ʿAbdullāh ibn Masʿūd (رضي الله عنه) was asked the meaning of the verse:

وَلَا تَحْسَبَنَّ الَّذِينَ قُتِلُوا فِي سَبِيلِ اللَّهِ أَمْوَاتًا ۚ بَلْ أَحْيَاءٌ عِندَ رَبِّهِمْ يُرْزَقُونَ ‎

“Do not think that those who were killed in Allah’s path are dead; rather, they are alive receiving provisions.”
[Ālī-ʿImrān, 3:169]

He responded: “We asked the Prophet (صلى الله عليه وسلم) about the meaning of this verse. The Prophet (صلى الله عليه وسلم) said: ‘The martyrs’ souls live in the bodies of green birds whose nests hang from the throne of The Almighty in chandeliers/lanterns. They eat the fruits of Paradise from wherever they like and then nestle in these chandeliers/lanterns.’” (Reported by Muslim (1887))

Because Saʿd ibn Muʿādh (رضي الله عنه) was so revered, the Prophet (صلى الله عليه وسلم) even spoke about him after his death. Al-Barāʾ (رضي الله عنه) relayed: “The Prophet (صلى الله عليه وسلم) was given a silk cloth. His Companions started touching it and admiring its softness. The Prophet (صلى الله عليه وسلم) said: ‘Are you admiring its softness? Saʿd ibn Muʿādh’s (رضي الله عنه) handkerchiefs [in Paradise] are better and softer than this.’” (Reported by al-Bukārī (3802))

Therefore, believing in the shaking of Allāh’s throne is a part of faith since it is from knowledge of the unseen. Were it not for the aforementioned authentic report, we would not have believed in this event or even known about it.