Section 121
Estoppel against denying capacity of payee to indorse
Chapter 13: Special rules of evidence
No maker of a promissory note and no acceptor of a bill of exchange [payable to order] shall, in a suit thereon by a holder in due course, be permitted to deny the payee's capacity, at the date of the note or bill, to indorse the same.
The Negotiable Instruments Act, 1881 (Act No. 26 of 1881). From the updated text on India Code, with later amendments. Being checked against the official copy; amendment notes are not shown yet.