Reference for Bava Kamma 159:1

והטבח לוקח ושוחט לוקח ושוהה ובלבד שלא ישהה העגונה שבהן ל' יום:

'A cattle dealer may, however, buy and slaughter, or buy and [even] keep for the market. He may, however, not retain the animal he bought last for thirty days.'

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