Reference for Niddah 73:20

מאי לאו לסתירה

Hence it renders void the counting of seven days. What had his emission of semen caused him? The [uncleanness of] one day. Hence it renders void the counting of one day'. Now what is meant by 'seven days'? If it be suggested that it causes him to be unclean for seven days, [the objection would arise that] in that case it should have been said: As on account of his <i>zibah</i> he is unclean for seven days. Consequently<span class="x" onmousemove="('comment',' Since the expression used was 'caused'. ');">¹⁷</span>

Jastrow

Jastrow