Gittin 31A

בשתי תרומות הכתוב מדבר: אחת תרומה גדולה ואחת תרומת מעשר; כשם שתרומה גדולה ניטלת באומד ובמחשבה, כך תרומת מעשר ניטלת באומד ובמחשבה; וכשם שיש לו רשות לבעל הבית לתרום תרומה גדולה, כך יש לו רשות לבעל הבית לתרום תרומת מעשר.

The verse speaks of two terumot; one is standard teruma and one is teruma of the tithe. Since they are mentioned together, they can be compared: Just as standard teruma is taken by estimate, as there is no requirement for the amount separated to be measured precisely, and it can be taken by thought, as one is not required to physically separate it before consuming the remaining produce, so too, teruma of the tithe may be taken by estimate and by thought. And this comparison also teaches that just as the homeowner has permission, i.e., the right, to separate standard teruma, so too, the homeowner has permission to separate teruma of the tithe.

מתני׳ המניח פירות, להיות מפריש עליהן תרומה ומעשרות; מעות, להיות מפריש עליהן מעשר שני ⁠– מפריש עליהן בחזקת שהן קיימין. אם אבדו ⁠– הרי זה חושש מעת לעת, דברי רבי אלעזר.

Mishnah: With regard to one who sets aside produce with the understanding that he will separate terumot and tithes with it, so that when he has untithed produce he can render it fit by declaring that the teruma and tithes that must be separated will be from the produce that he had set aside for this purpose, or one who sets aside money with the understanding that he will separate and redeem second tithe with it, then he may later separate the teruma or tithe with them, based on the presumption that the produce or the money are extant. He need not be concerned that perhaps the produce or money was lost in the meantime. If he discovers that they were lost, then he must be concerned that the produce or money that he set aside was lost, from the time until the same time, as will be explained in the Talmud, and he must separate teruma and tithes from the produce a second time; this is the statement of R' Elazar.