Shiva Purana
यथांतर्विष्ठया पूर्णश्शुचिमान्न बहिर्घटः ॥ शोध्यमानो हि देहोऽयं तेनायमशुचिस्ततः ॥ ३ ॥
Just as a pot, full of faeces cannot become pure even when washed outside, so also the body washed outside cannot become pure. Hence it is impure.
english translation
yathAMtarviSThayA pUrNazzucimAnna bahirghaTaH ॥ zodhyamAno hi deho'yaM tenAyamazucistataH ॥ 3 ॥
hk transliteration by Sanscript