Sa'id bin al-Musayyib (may Allah have mercy on him) said: There is no usury (riba) in animals; rather, three things concerning animals are forbidden: 1. al-madamin, 2. al-malaqih, 3. habal al-habalah. Al-madamin is what is in the bellies of females, and al-malaqih is what is in the loins of male camels.
The Shaykh (may Allah have mercy on him) said: Imam al-Muzani (may Allah have mercy on him) narrates from Imam al-Shafi'i (may Allah have mercy on him) that al-madamin is what is in the loins of male camels, and al-malaqih is what is in the bellies of females.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10863