The Qur'an speaks of a tree it calls the "lotus tree" or "lote-tree of the uttermost boundary," the sidrat al-muntaha. The reference to what Muhammad saw, quoted here, traditionally refers to the Prophet's mi'raj, or ascent to heaven. During this ascent he saw "the Heavens and the Lote Tree, the Divine Throne and [the] Divine Court."
To some classical commentators, "It is called 'Sidratul Muntaha' (the lotus tree of the extreme limit) because it is the end of all that is knowable, and nobody can know what is beyond it."
From Hajjah Amina Adil, Muhammad, the Messenger of Islam: His Life and Prophecy, 155, 219.