Musar for Yevamot 149:4

ואי כתב רחמנא עד מלאת הוה אמינא אפילו בלא טבילה כתב רחמנא עד אשר יטהר

it might have been assumed [to apply to such a person] as is not liable to bring a sacrifice, but in the case of one who is liable it might have been presumed that cleanness is not effected before he has brought his atonement, hence the All Merciful wrote, Until … be fulfilled.<span class="x" onmousemove="('comment',' Ibid. XII, 4. which speaks of a woman after childbirth, who is liable to bring a sacrifice and is, nevertheless, regarded as clean in respect of terumah immediately after the sunset of the last day of the prescribed period. ');">⁹</span> And had the All Merciful written only, Until&nbsp;… be fulfilled,<span class="x" onmousemove="('comment',' V. supra n. 24. ');">¹⁰</span> it might have been presumed that cleanness may be effected even without ablution, hence did the All Merciful write, Until he be clean.<span class="x" onmousemove="('comment',' Lev. XXII, 4. ');">⁶</span>

Shenei Luchot HaBerit