Reference for Nedarim 15:4

הא קמ''ל כיון דאי בעי פטר נפשיה בקרית שמע שחרית וערבית משום הכי חייל שבועה עליה

R. Giddal also said in Rab's name: He who says, 'I will rise early to study this chapter or this tractate,' has vowed a great vow to the God of Israel. But he is under a perpetual oath from Mount Sinai, and an oath cannot fall upon another?<span class="x" onmousemove="('comment',' I.e., an oath is not valid when referring to that which is already subject to an oath. ');">⁴</span>

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