Responsa for Bava Batra 308:11

ברם נראין דברי רבינו יוסף דאמר רבינו יוסף אמר רב יהודה אמר שמואל זו דברי חכמים אבל רבי מאיר אומר מודה בשטר שכתבו שצריך לקיימו ומאי דברי הכל דרבנן לגבי רבי מאיר דברי הכל היא

R. Zeira said: If R. Johanan could contradict his disciple R. Eleazar,<span class="x" onmousemove="('comment',' Who reported in his name. ');">³⁷</span> would he contradict his master R. Jannai? For R. Jannai said in the name of Rabbi: [Though] one admits that he wrote a deed, attestation is [nevertheless] required. And R. Johanan said to him: 'Is not this, Master, [the law enunciated in] our Mishnah [where it is stated] AND THE SAGES SAY: HE WHO CLAIMS FROM THE OTHER HAS TO PRODUCE THE PROOF, [and] proof [can be produced] only through the attestation of the deed?'<span class="x" onmousemove="('comment',' Which clearly proves that, according to R. Johanan, the Sages require attestation even when the authenticity of a deed had been admitted. ');">³⁸</span> Acceptable, however, are the words of our master Joseph. For our Master Joseph, in the name of Rab Judah in the name of Samuel, said: 'This<span class="x" onmousemove="('comment',' That no attestation is needed when the giver of the deed had admitted writing it, ');">³⁹</span> is the view of the Sages. but R. Meir said: [Though] one admits the writing of a deed, attestation is [nevertheless] required;<span class="x" onmousemove="('comment',' Thus it is the Sages, and not R. Meir, who require no attestation, when the writing of a deed had been admitted. ');">⁴⁰</span>

Teshuvot Maharam