التفسير التبليغي
(23) And indeed, he saw him in the clear horizon.
That is, the highest [horizon] which is at the Lote Tree of the Utmost Boundary (Sidrat al-Muntaha).
Angels are subtle, luminous bodies. Allah has veiled them from the sight of humans, so they do not see them, and He has veiled our sight from seeing them. No one from this nation has seen the angels in their true forms except the Messenger (peace be upon him), just as he saw Jibreel twice. Humans can see angels if the angels manifest themselves in the form of a human.
Source: التفسير التبليغي