BṚHADĀRAṆYAKA UPANISHAD · 6.4.13

अथ यस्य जायाम् आर्तवं विन्देत् त्र्यहं कंसे न पिबेत् ।
अहतवासाः ।
नैनां वृषलो न वृषल्य् अपहन्यात् ।
त्रिरात्रान्त आप्लुत्य व्रीहीन् अवघातयेत् ॥

atha yasya jāyām ārtavaṃ vindet tryahaṃ kaṃse na pibet ।
ahatavāsāḥ ।
naināṃ vṛṣalo na vṛṣaly apahanyāt ।
trirātrānta āplutya vrīhīn avaghātayet ॥

Now, when the monthly sickness comes upon anyone's wife, for three days she should not drink from a metal cup, nor put on fresh clothes. Neither a low-caste man nor a low-caste woman should touch her. At the end of the three nights she should bathe and should have rice threshed.

Robert Ernest Hume · The Thirteen Principal Upanishads (1921), Bṛhadāraṇyaka VI.4