Maʿānī al-ʾAkhbār > The meaning of haqila, muzabana, ‘araya, mukhabarat, mukhadhara, munabadha, mulamasa, the selling of pebble and other things that were declared forbidden
Hadith #4

ونهى صلى الله عليه وآله عن المجر وهو أن يباع البعير أو غيره بما في بطن الناقة. ويقال: منه أمجرت في البيع إمجارا.

He, Allah’s blessings be upon him and his Family, also forbade majr, and that is to sell a camel or other with what is in the belly of a female camel. It is said: «One performed imjar in the sale, a (true act of) imjar.»