AITAREYA UPANISHAD · 1.3.9

तच्छिश्नेनाजिघृक्षत् तन्नाशक्नोच्छिश्नेन ग्रहीतुं स यद्धैनच्छिश्नेनाग्रहैष्यद् विसृज्य हैवान्नमत्रप्स्यत् ॥ ९ ॥

tacchiśnenājighṛkṣat tannāśaknocchiśnena grahītuṃ sa yaddhainacchiśnenāgrahaiṣyad visṛjya haivānnamatrapsyat ॥ 9 ॥

By the organ He would have seized it, but He could not seize it by the organ. Had He seized it by the organ, then would a man be satisfied by merely emitting food.