"An hour" is correct in English as the "h" is not pronounced. For example, we say "a hospital" but "an hour." There are English speakers who will say "an hospital" and it is also correct but I "a hour" is not correct.

I stand corrected. An hour is not an instant is correct English grammar.

Typo error: It tells me, "an hour is not an instant". An comes before a vowel. "H" is not a vowel.

my answer was not wrong.It had a spelling mistake in one of the words.