Tractate Shevuot - Chapter 8 - Mishnah 6

[If the owner said,] “Where is my ox?” and he replied to him, “I do not know what you are talking about,” whereas in reality it died or was injured or captured or stolen or lost; [and the owner said,] “I adjure you,” and he said, “Amen,” he is liable. If he said to a paid guardian, or hirer, “Where is my ox?” and he replied to him, “It died,” whereas in reality it was injured or captured; [Or he replied,] “It was injured,” whereas in reality it died or was captured; [Or he replied,] “It was captured,” whereas in reality it died or was injured; [Or he replied,] “It was stolen”, whereas in reality it was lost; [Or he replied,] “It was lost,” whereas in reality it was stolen; [And the owner said,] “I adjure you,” and he said, “Amen,” he is exempt. [If he replied,] “It died,” or, “It was injured,” or, “It was captured,” whereas in reality it was stolen or lost; [And the owner said,] “I adjure you,” and he said, “Amen,” he is liable. [If he replied,] “It was lost,” or, “It was stolen,” whereas in reality it died or was injured or captured; [And the owner said,] “I adjure you,” and he said, “Amen,” he is exempt. This is the principle: he who [by lying] changes from liability to liability or from exemption to exemption, or from exemption to liability, is exempt; From liability to exemption, is liable. This is the principle: he who takes an oath to make it more lenient for himself, is liable; to make it more stringent for himself, is exempt.