Talmud for Pesachim 79:16
מ"ט משום דלא עבד בהו שימור ולעביד ליה שימור מאפייה ואילך אלא לאו שמע מינה שימור מעיקרא בעינן
Now, if not that it requires washing, for what purpose is the guarding?<span class="x" onmousemove="('comment',' For the grain cannot ferment unless there is moisture upon it.');"><sup>25</sup></span> If guarding for the kneading.<span class="x" onmousemove="('comment',' I.e., that when it is kneaded care must be taken that it does not turn leaven.');"><sup>26</sup></span> the guarding of kneading is not guarding,<span class="x" onmousemove="('comment',' This verse implies that at a certain stage of its manufacture the unleavened bread must be guarded for the express purpose of fulfilling the law prescribing the eating of unleavened bread. Hence, if a man eats on the first night of Passover only unleavened bread which was not guarded expressly for that purpose, he does not do his duty. Now Raba states that the guarding that is given to it at the stage of kneading is not considered 'guarding' in this respect.');"><sup>27</sup></span>