The song is from season 3 episode 13 of MLP:FiM.
♫ Lyrics for the song:
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[Twilight Sparkle]

Life in Equestria shimmers
Life in Equestria shines
And I know for absolute certain

[Main Cast]

That everything (everything)
Yes, everything
Yes, everything is certainly fine
It's fine

Twilight Sparkle: Yes! Everything's going to be just fine!