The story
A corruption of German mathematician Leopold Kronecker's finite dictum “die ganzen Zahlen hat Gott gemacht; alles andere ist Menschenwerk” : “God made the integers; everything else is the work of man”. The reason for the addition of an "s" to "andere" ("anderes" is grammatically incorrect and not present in the German phrase) is unclear.
Etymology from Wiktionary, CC BY-SA 4.0.