Reference for Chullin 165:56

<big><strong>מתני׳</strong></big> (ויקרא כב, כח) יום אחד האמור באותו ואת בנו היום הולך אחר הלילה

I might have thought that here also it is the same, it is therefore writte here 'one day' and also 'one day' in connection with the Creation, as the 'one day' mentioned in connection with the Creation means the day and the night preceding it, so, too, the 'one day' mentioned in connection with the law of 'It and its young' means the day and the night preceding it.

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