Reference for Nedarim 55:12
תנא מתניתין בנדר להודיעך כחן דבית שמאי תנא ברייתא בשבועה להודיעך כחן דבית הלל
R. Huna said: A Tanna taught: Beth Shammai maintain: He must not volunteer with an oath; Beth Hillel say: He may volunteer even with an oath. Now, in the view of Beth Shammai, only with an oath may he not volunteer, but he may volunteer a vow. But we learnt: BETH SHAMMAI RULE: THE OWNER MUST NOT VOLUNTEER TO VOW. Moreover, he may merely not volunteer an oath, but he may vow with an oath [if requested]; but we learnt, BETH SHAMMAI MAINTAIN: ONE MAY MAKE ANY FORM OF VOW, EXCEPTING THAT SUSTAINED BY AN OATH? — The Mishnah deals with a vow, to shew how far-reaching is Beth Shammai's ruling;<span class="x" onmousemove="('comment',' I.e., one may not volunteer even a vow, which is not as grave as an oath. ');"><sup>8</sup></span>