Reference for Shevuot 51:5
שפיר קא"ל רבי עקיבא לר' ישמעאל
Our Rabbis taught: [Whatsoever it be that] a man [shall utter clearly] with an oath - this excludes [a false oath by] accident; and it be hid - this excludes wilful [transgression of oath]; from him - [this implies that] the oath was hidden from him. I might think that [even] if the thing be hidden from him [he should be liable], therefore it is said: . . . with an oath, and it be hid ... for the unawareness of the oath he is liable, and he is not liable for the unawareness of the thing. The Master said: '.. . a man . . . with an oath - this excludes [a false oath by] accident'. How is this? As the case of R. Kahana and R. Assi: when they rose from [the lecture of] Rab, one said, 'I swear that thus said Rab,' and the other said, 'I swear that thus said Rab.' When they came [again] before Rab, he would agree with one of them; then the other would say to him, 'Did I, then, swear falsely?' He would reply to him, 'Your heart deceived you.' <br>