The Mukhya Upaniṣads

Maitrī · Prapāṭhaka 3 · Maitrī 3.1

Passage 4

अथान्यत्राप्युक्तं शरीरमिदं मैथुनादेवोद्भूतं संविदपेतं निरय एव मूत्रद्वारेण निष्क्रामन्तमस्थिभिश्चितं मांसेनानुलिप्तं चर्मणावबद्धं विण्मूत्रपित्तकफमज्जामेदोवसाभिरन्यैश्च मलैर्बहुभिः परिपूर्णं कोश इवावसन्नेति ॥३.४ ॥

In plain words

And it is said elsewhere too: This body has arisen from coupling, has grown up bereft of awareness in what is a very hell, and has come out by the passage of urine. It is built of bones, smeared with flesh, bound about with skin, and filled full with dung and urine and bile and phlegm and marrow and fat and grease and many other impurities — like a treasure-chest packed with them.