Reference for Bekhorot 63:18

לא יגאל ואינו נמכר לא חי ולא שחוט ולא תם ולא בעל מום

The [above] text states: In connection with a firstling Scripture says: 'Thou shalt not redeem', implying th it may be sold when alive, and in connection with tithing, it is said in the Scriptures: 'It shall not be redeemed', intimating that it is forbidden to be sold either alive or ritually cut, whether unblemished or blemished.

Jastrow