Responsa for Pesachim 79:19

מר בריה דרבינא

Then let him guard it from the baking and onwards?<span class="x" onmousemove="('comment',' I.e., from when it is prepared for baking. viz., when it is shaped, moistened and put into the oven.');">³¹</span> Hence this surely proves that we require guarding from the beginning. Yet whence [does this follow]: perhaps it is different there, because when guarding became necessary.<span class="x" onmousemove="('comment',' Lit., 'when it entered upon (the need for) guarding'. - I.e., at the beginning of the kneading process - from the moment when water was added to the flour making fermentation possible.');">³²</span> he did not guard it;<span class="x" onmousemove="('comment',' Though it nevertheless remained unleavened.');">³³</span> but where he did guard it when guarding became necessary. it may indeed be that the guarding at the kneading is [truly] considered 'guarding'. Yet even so,<span class="x" onmousemove="('comment',' Though Raba's proof was refuted.');">³⁴</span> Raba did not retract. For he said to those who handled sheaves,<span class="x" onmousemove="('comment',' At harvest time, gathering and tying them. Lit., 'turned about'.');">³⁵</span> Handle them for the purpose of the precept.<span class="x" onmousemove="('comment',' Bear in mind that they may be used for that purpose.');">³⁶</span> This proves that he holds [that] we require guarding ab initio, from beginning to end. Mar the son of Rabina,

Teshuvot Maharam