Welcome, Christmas, bring your cheer.
Cheer to all Whos far and near.
Christmas Day is in our grasp,
so long as we have hands to clasp.
Christmas Day will always be
just as long as we have we.
Welcome Christmas while we stand,
heart to heart, and hand in hand.

Dr. Seuss (1904-1991) American author, illustrator [pseud. of Theodor Geisel]

How the Grinch Stole Christmas!, TV special, final lines [Narrator] (1966) [with Irv Spector, Bob Ogle, Eugene Poddany]
    (Source)

This phrase was from the 1966 animated special; it does not appear in the original 1957 book. It is unclear who actually wrote the words to the song, but "Dr. Seuss" is sually credited. The words are also embedded in the Whos' "Welcome Christmas" song. The narration was performed by Boris Karloff.

The last four lines are often paraphrased down to "Christmas will always be as long as we stand heart to heart and hand in hand."