The term "al-Kitāb" (الكتاب) in Surah Al-Baqarah, Ayah 2, refers to the Qur'an itself.

Ibn Kathir clarifies that "al-Kitāb" here unequivocally means the Qur'an, and "rayb" (ريب) signifies doubt. This interpretation is supported by numerous Companions, including Ibn `Abbas and Ibn Mas`ud, with Ibn Abi Hatim noting no disagreement on this explanation. The meaning conveyed is that the Qur'an is undeniably revealed from Allah. This is further reinforced by similar phrasing in Surat As-Sajdah (32:2): "The revelation of the Book (this Qur'an) in which there is no doubt, is from the Lord of all that exists."

Linguistically, "al-Kitāb" is the definite form of "kitāb" (book), indicating a specific and well-known scripture. The definite article "al-" (ال) emphasizes its unique status.

The consensus among early scholars, as reported by Ibn Kathir, is that "al-Kitāb" in this context refers to the divine revelation of the Qur'an, free from any doubt regarding its origin and authenticity.

📚 Sources: Classical tafsir (multi-source)