My Twitter: http://goo.gl/yTQcO7 | The song is from season 2 episode 18 of MLP:FiM.
♫ Lyrics for the song:
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[Pinkie Pie]

Welcome welcome welcome
A fine welcome to you
Welcome welcome welcome
I say how do you do?
Welcome welcome welcome
I say hip hip hurray
Welcome welcome welcome
To Ponyville today

Pinkie Pie: Wait for it!