Reference for Shevuot 39:1

מסרבין בו לאכול

<br> urged to eat: our Mishnah [refers to the case where] he is not urged to eat; and the Baraitha [to the case where] he is urged to eat, and he says: 'I shall not eat, I shall not eat'; so that when he swears, he means: 'I swear I shall not eat'. R. Ashi said: Read [in the Baraitha]: 'I swear I shall not eat of thine'. If so, what need is there to state it? - I might have thought his tongue became twisted, therefore he teaches us [that it is a definite negative].

Jastrow

Jastrow

Jastrow