Harry, Niall and Liam were all backstage with Louis at the show, where he performed his new single for the first time, in tribute to his late mother Johannah Deakin, who passed away earlier this week after a secret battle with leukaemia.
The group have been on hiatus for almost a year but the trio dropped everything to be with Louis after he decided to go ahead with the performance, which had been planned for some time and which his mother had been looking forward to.
A source told MailOnline: "Harry, Niall and Liam all came down to X factor to support Louis. They all were backstage with him supporting him. It was a lovely moment - and shows the unity of 1D. First time all back together in a year.
"It was a lovely touching moment backstage and they all came to support Louis during a horrible time. Niall even flew back from New York."
However, Louis admitted getting up on stage just days after Johannah's death, was "harder than I ever imagined".
He tweeted: "That was harder than I ever imagined. I want to thank everyone around me and all of the amazing fans out there that made that so special!
