Bukhari

Hadith Commentary: Hafiz Ibn Hajar said: Musa ibn Uqbah narrated that the flood used to come towards the Ka'bah from the direction of the mountain which is on the elevated side. They feared that water might enter the Ka'bah, so they wanted to reinforce the structure firmly. The first person who raised the Ka'bah and demolished part of it was Dalid ibn Mughirah.

Then he narrated the story of the construction of the Ka'bah that took place before the Prophethood of the Messenger of Allah (sallallahu alayhi wa sallam). And Imam Shafi'i, in his book "al-Umm," transmitted from Abdullah ibn al-Zubayr (radi Allahu anhu) that when he was constructing the Ka'bah, Ka'b said to him: "Build it strong, for we find in the books that in the end times there will be many floods."

So, the meaning of the story is that, upon witnessing this flood—which was unprecedented—they understood that this was the first of the floods of the end times.

Hadith Commentary:
➊
Floods in Makkah Mukarramah used to come from the direction of the mountain located on the upper side. Once, when a flood came, the large wall situated in the upper part of Makkah was swept away. People felt threatened that water might enter the Baytullah, so they intended to strengthen its foundations. The first person who looked inside it and demolished a part of it was Walid bin Mughirah.

➋
The flood and the reconstruction of the Baytullah occurred before the Prophetic mission (bi‘thah) of the Prophet sallallahu alayhi wa sallam.

➌
The People of the Book used to find in their scriptures that in the end times, there would be many floods. In the Age of Ignorance (Jahiliyyah), when the flood came, they thought that this was the first of the floods of the end times.

➍
In any case, the reconstruction of the Baytullah took place during the Age of Ignorance. The great event referred to in the hadith is this very incident. (Fath al-Bari: 7/189) And Allah knows best.