Sa'id bin al-Musayyib (may Allah have mercy on him) said: There is no usury in animals; but in animals the following are forbidden: (1) al-madamin (the young in the female's womb), (2) al-malaqih (the camel's mating), (3) habal al-habalah (that a female gives birth, and then that offspring becomes pregnant and gives birth).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10525