Reference for Gittin 151:2

אלא לרב אשי מאי שנא רישא ומאי שנא סיפא קשיא

speaks of the case where he does not mention any time limit and this where he does.<span class="x" onmousemove="('comment',' [For the fact of his mentioning a time limit shows that he is particular about the child being suckled for two years. So Rashi; but v. Tosaf.] ');">²</span>

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