Tosefta על קידושין 108:21

[Someone who] was selling produce during Shevi'it (i.e., the seventh year of the agriculture cycle, when fields must lie fallow) and said that they [were grown] just before [the start of] Shevi'it, [that produce is] liable for tithing and for Bi'ur (i.e., the act of publicly renouncing and only then reacquiring ownership over produce grown during Shevi'it, see Kidd. 54b:7). [Someone who] was selling produce right after [מוצאי not ערב] Shevi'it and said that it [was grown] after Shevi'it, [that produce] is exempt from tithing and exempt from Bi'ur. [The rule is that in a situation where] a matter about which an exemption [is derived from] a person's silence, his stringency is [also] believed; and therefore, his assertion of leniency [is also believed and renders the matter] exempt [see Y. Dem. VI.8.2].