Reference for Menachot 85:7

רבי מני דייק וזבין כחומרי מתניתא א"ל ההוא סבא הכי עבוד קמאי דקמך ואצלח עיסקייהו

whereupon a certain old man said to him, Those who long preceded you acted so, and they were successful in their business. Our Rabbis taught: If a man bought a garment furnished with zizith from an Israelite in the market, the presumption is [that it is valid];<span class="x" onmousemove="('comment',' I.e., the blue thread in the zizith is deemed to be genuine.');">⁸</span>

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