Square Pyramidal Number also Square

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