Reference for Bekhorot 116:40
(במדבר יח, כז) ונחשב לכם תרומתכם בשתי תרומות הכתוב מדבר אחת תרומה גדולה ואחת תרומת מעשר כשם שהתרומה גדולה ניטלת באומד ובמחשבה
I have here mentioned only that the tenth animal is holy when he calls it the tenth. Whence is it derived that it is holy even if he did not call it tenth? Scripture says: 'It shall be holy', intimating that [it is holy] in any case. You might think that if he a hundred [lambs] and he took ten [at the same time as the tithe], or if he had ten lambs and he took one [as the tithe], they are redeemed? The text states: 'The tenth', and this is not the tenth.<span class="x" onmousemove="('comment',' Not having been counted.');"><sup>26</sup></span> But R'Jose son of R'Judah says: Such is [valid] tithe. What is the reason of R'Jose son of R'Judah? He agrees with Abba Eleazar B'Gomel.<span class="x" onmousemove="('comment',' Or, Gamala; v. Bez. 13b, Git. 30b.');"><sup>27</sup></span> For it was taught: Abba Eleazar B'Gomel Says: [Scripture says]: And this your heave-offering shall be reckoned unto you as though it were the corn of the threshing-floor.<span class="x" onmousemove="('comment',' Num. XVIII, 27.');"><sup>28</sup></span> Scripture speaks of two kinds of terumah,<span class="x" onmousemove="('comment',' The verse refers to the Levite who has to give terumah to the priest from the tithe he received from the Israelite and this is compared to the terumah which the Israelite gives to the priest from the threshing-floor, v. Glos. s.v. Terumah.');"><sup>29</sup></span> one that of terumah gedolah<span class="x" onmousemove="('comment',' The gift which the Israelite gives to the priest is called the 'great' because it is the first to be separated from the grain. cajbu');"><sup>30</sup></span> and the other the terumah of the tithe.<span class="x" onmousemove="('comment',' The text: 'And this shall be reckoned unto you , referring to the gift of terumah given by the Levite to the priest may also be held to refer to the terumah given by the Israelite to the priest.');"><sup>31</sup></span> Just as terumah gedolah may be set apart for the priest by estimating [without measuring the quantity]<span class="x" onmousemove="('comment',' Estimating approximately how many se'ah there are, and he gives terumah according to his judgment.');"><sup>32</sup></span> and by [merely] mentally planning [the separation],<span class="x" onmousemove="('comment',' And not actually separating with the hand but merely glancing at one side and deciding (Lit., 'thinking') to give it as terumah, after which he can immediately eat what is on the other side.');"><sup>33</sup></span>