Reference for Chullin 271:51

ראשית אין לך בו אלא ראשית בלבד

Again just as in the case of terumah one must not set aside new [grain as terumah] for old<span class="x" onmousemove="('comment',' The produce of one year may not be given as terumah or tithe for the produce of the preceding year, or vice versa, for it is written: That which is brought forth in the field year by year (Deut. XIV, 22) .');">³⁶</span>

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