The song is from season 9 episode 3 of MLP:FiM.
♫ Lyrics for the song:
( You can support the show here: https://itunes.apple.com/us/tv-season... )
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[Silverstream]

We celebrate our friendship

[Ocellus]

We celebrate the tree

[Sandbar]

The memories inside us

[Young Six]

We'll build for all to see

[Smolder]

The tree brought us together

[Ocellus, Silverstream, and Gallus]

(Brought us together)

[Smolder]

And even though it's gone

[Ocellus, Silverstream, and Gallus]

(Even though it's gone)

[Gallus]

Our monument of friendship

[Rest of Young Six]

(Monument of friendship)

[Young Six]

Can make the gift live on

[Ocellus]

A piece from me, a piece from you

[Silverstream]

We'll use the tree and we'll make do

[Gallus]

If things don't fit, there's lots of glue

[Young Six]

Friends don't quit in the end
And together, we'll see this through

[Smolder]

It's hard to say farewell

[Sandbar]

To something you hold dear

[Young Six]

But goodbye isn't forever
As long as we are here

The tree is in our memory (ahh-ahh-ahh)
Its roots have grown deep in our hearts
(Deep in our hearts, deep in our hearts)
Its branches raise our spirits, together we're all bound
Forever it stands, lost but now it's found

[Smolder]

So lift a beam and build it strong

[Silverstream]

Work as a team, it won't take long

[Yona]

Hey, Silverstream, I think that's wrong

[Young Six]

We'll live the dream and sing our song

And make the tree something all can see
In a place where we belong!

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"My Little Pony" is owned by Hasbro,
"MLP: Friendship is Magic" is created by Lauren Faust.