Tosefta for Avodah Zarah 78:9

[If someone] was among a group of people traveling together, and said, "Who here is trustworthy?" [or] "Who here tithes?" (Dem. 4:6), and one said to him, "I am trustworthy" [or] "He (i.e., another person) tithes" (alt., "I am not trustworthy" [or] "he tithes" (MS Vienna)), behold, this one is believed. [If] he said to him, "He tithes" in front of him (i.e., in front of the person who he claims tithes), he is believed, and if not in front of him, he is not believed. In what case does this apply? In a situation that [the traveler] does not know anyone there, but if he did know someone there, he should not take from anyone except an expert (i.e., one who has a reputation for knowing and upholding halachah, see Avodah Zarah 39b:6). In what [other] case does this apply? In a situation where he has not stayed there for 30 days, but [if] he has stayed there for 30 days, he should not take from anyone other than a specialist. In what [other] case does this apply? With respect to Terumah and tithes [he may rely on an unverified statement of trustworthiness], but with respect to [the laws of] seventh-year produce and [whether foods were prepared in a state of] purity, he is not believed.