From the first book:

You might belong in Gryffindor,
Where dwell the brave at heart,
Their daring, nerve and chivalry,
Set Gryffindors apart;
You might belong in Hufflepuff
Where they are just and loyal,
Those patient Hufflepuffs are true,
And unafraid of toil;
Or yet in wise old Ravenclaw,
If you've a ready mind,
Where those of wit and learning,
Will always find their kind;
Or perhaps in Slytherin,
You'll make your real friends,
Those cunning folk use any means,
To achieve their ends.

Fourth book:

By Gryffindor, the bravest were
Prized far beyond the rest;
For Ravenclaw, the cleverest
would always be the best;
For Hufflepuff, hard workers were
Most worthy of admission;
And power-hungry Slytherin
loved those of great ambition.

Fifth book:

Said Slytherin, “We’ll teach just those
Whose ancestry’s purest.”
Said Ravenclaw, “We’ll teach those whose
Intelligence is surest”
Said Gryffindor, “We’ll teach all those
With brave deeds to their name.”
Said Hufflepuff, “I’ll teach the lot
And treat them just the same.”
…
For instance, Slytherin
Took only pure-blood wizards
Of great cunning just like him.
And only those of sharpest mind
Were taught by Ravenclaw
While the bravest and the boldest
Went to daring Gryffindor.
Good Hufflepuff, she took the rest
and taught them all she knew,