He posted: "Am listening to the Alex and Sierra demos for their upcoming album. Fantastic. It's seriously good. I can't wait for everyone to hear it."
Lyrics for the romantic ballad include: "I saw your face, I heard your name / Felt the blood rushing through my veins / I still remember / How could I forget?"
A source added: "It's a testament to Harry's talents that he can work alongside John - who is, without doubt, one of the most gifted songwriters of a generation."
Harry, who is still focused on work with his band mates Zayn Malik, Louis Tomlinson, Liam Payne and Niall Horan, is credited to 26 songs on the American Society of Composers, Authors and Publishers database - 18 of which have never been heard before.
American indie pop duo Alex Kinsey and Sierra Deaton won the third and final season of the US version of The X Factor and were mentored by Simon Cowell.
They were the only group ever to win the American show.
