Indeed, the pinnacle of the Qur'an is Surah Al-Baqarah, and in it there is a verse that is the master of all the verses of the Qur'an: it is the Ayat al-Kursi. (1)

This is a strange hadith; we do not know it except from the narration of Hakim ibn Jubayr. Shu'bah has spoken critically of Hakim ibn Jubayr and declared him weak.

3095 - Yahya ibn al-Mughirah Abu Salamah al-Makhzumi al-Madini narrated to us, saying: Ibn Abi Fudayk narrated to us, from Abd al-Rahman al-Mulayki, from Zurarah ibn Mus'ab, from Abu Salamah, from Abu Hurayrah, who said: The Messenger of Allah, peace and blessings be upon him, said: "Whoever recites Ha Mim al-Mu'min up to {To Him is the final return} [Ghafir: 1-3] and the Ayat al-Kursi in the morning, he will be protected by them until the evening. And whoever recites them in the evening, he will be protected by them until the morning."

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(1) Its chain of transmission is weak, due to the weakness of Hakim ibn Jubayr.
It was also recorded by Abd al-Razzaq (6019), al-Humaydi (994), Ibn Adi in "al-Kamil" 2/637 (in the entry on Hakim ibn Jubayr), al-Hakim 1/560 and 2/259. Al-Hakim declared it authentic and said: "The two Shaykhs did not narrate from Hakim ibn Jubayr due to weakness in his narrations; they only abandoned him because of his extremism in Shi'ism."
The first part of it is supported by the hadith of Sahl ibn Sa'd, recorded by Abu Ya'la (7554) and al-Tabarani in "al-Kabir" (5864) and (8644). It is also found with Ibn Hibban (780), and its chain is weak.
The report of Abdullah ibn Mas'ud, as a saying attributed to him (mawquf), was recorded by al-Darimi (3377), al-Tabarani in "al-Kabir" (8644), and al-Hakim 2/259, and its chain is good (hasan).
His saying: "For everything there is a pinnacle (sanam)": it is with a fathah on the sin, meaning its height and elevation, borrowed from the hump of a camel. Then its usage became frequent in this sense until it became proverbial, and from this Al-Baqarah was called the pinnacle of the Qur'an. Al-Tibi said this.