Square Pyramidal Number also Square

Theorem
$4900$ is the only square pyramidal number which is also square:
 * $4900 = 70^2 = \displaystyle \sum_{k \mathop = 1}^{24} k^2 = \dfrac {24 \paren {24 + 1} \paren {2 \times 24 + 1} } 6$