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This verse is said to exist in the Vedas:

yad vai manuravadat tad bheṣajam

‘Whatever Manu has said is medicine.’

Krishana Yajurveda ➡ Taittariya Samhita : 2.2.10.2

Quoting Sanskrit text from Wikisource: Taittariya Samhita, Adhyaya-2, Prapathaka-2, Anuvaka-10, 2^nd Mantra:

VERSE: 2

वै तिष्यः सोमः पूर्णमासः साक्षाद् एव ब्रह्मवर्चसम् अव रुन्द्धे
परिश्रिते याजयति ब्रह्मवर्चसस्य परिगृहीत्यै
श्वेतायै श्वेतवत्सायै दुग्धम् मथितम् आज्यम् अभवत्य् आज्यम् प्रोक्षणम् आज्येन मार्जयन्ते यावद् एव ब्रह्मवर्चसं तत् सर्वं करोत्य् अति ब्रह्मवर्चसं क्रियत इत्य् आहुः ।
ईश्वरो दुश्चर्मा भवितोर् इति मानवी ऋचौ धाय्ये कुर्याद् यद् वै किं च मनुर् अवदत् तद् भेषजम् ।

You can refer Krishna Yajurveda Taittariya Samhita in authentic/original format from Vedic Reserve, 2.2.10: (see pg. 105):

Transliteration from Sanskritweb.net:

[[2-2-10-2]]
vai tiÍyaÏ somaÏ pÂrÉamÀsaÏ sÀkÍÀd eva brahmavarcasam ava runddhe
pariÌrite yÀjayati brahmavarcasasya parigÃhÁtyai
ÌvetÀyai ÌvetavatsÀyai dugdham mathitam Àjyam abhavaty Àjyam prokÍaÉam Àjyena
mÀrjayante yavad eva brahmavarcasaÎ tat sarvaÎ karoty ati brahmavarcasaÎ
kriyata ity Àhus |
ÁÌvaro duÌcarmÀ bhavitor iti mÀnavÁ Ãcau dhÀyye kuryÀd yad vai kiÎ ca manur
avadat tad bheÍajam ||

The English translation from Sanskritweb.net:

[[2-2-10]]
Yonder sun did not shine, the gods sought an atonement for him, for him they offered this oblation
to Soma and Rudra: verily thereby they bestowed brightness upon him. If he desires to become
resplendent, he should offer for him this oblation to Soma and Rudra; verily he has recourse to
Soma and Rudra with their own portion; verily they bestow upon him splendour; he becomes
resplendent. He should offer on the full moon day of the month Tisya; Tisya is Rudra 1, the full
moon is Soma; verily straightway he wins splendour. He makes him sacrifice on an enclosed (altar),
to acquire splendour. The butter is churned from milk of a white (cow) with a white calf; butter is
used for the sprinkling, and they purify themselves with butter; verily he produces whatever
splendour exists. 'Too much splendour is produced', they say, 'he is liable to become a leper'; he
should insert the verses of Manu's; whatever Manu said is medicine [2]