5.6 The Pawn.
(a)
The pawn may move only forward [except as limited by Article 4.2].
(b)
Except when making a capture, it advances from its original square
either one or two vacant squares along the file on which it is placed,
and on subsequent moves it advances one vacant square along the
file. When capturing, it advances one square along either of the
diagonals on which it stands.
(c)
A pawn, attacking a square crossed by an opponent's pawn which has
[just] been advanced two squares in one move from its original
square, may capture this opponent's pawn as though the latter had
been moved only one square. This capture may only be made in
[immediate] reply to such an advance, and is called an "en passant"
capture.
(d)
On reaching the last rank, a pawn must immediately be exchanged,
as part of the same move, for [either] a queen, a rook, a bishop, or a
knight, of the same colour as the pawn, at the player's choice and
without taking into account the other pieces still remaining on the
chessboard. This exchange of a pawn for another piece is
called "promotion", and the effect of the promoted piece is immediate
[and permanent!].
(e)
In a competition, if a new piece required for the promotion is not
immediately available, the player may stop his clock to ask for the
assistance of the arbiter. The player must complete his move
correctly, in the manner specified in Article 5.6(d).
This is copied from FIDE. Paragraph (c) covers en pessant. I found it
using the google search engine, by entering "fide official rules" in the
search window.
regards, Marc