The evidence for determining the amount of Zakat-ul-Ghanam by Fuqaha', i.e., Jadh'ah and Thanny, is inherent in the Hadith narrated by
Suwayd ibn Ghaflah (may Allah be pleased with him), who said:
One of the Sahabah (Sahabah (Companions of the Prophet) came to us, saying: (Part No. 8; Page No. 15) 'We are ordered to take Jadh'ah and Thanny.'
Ahmad in his Musnad (Hadith compilation),
Al-Bayhaqy) This is because they are considered the minimum acceptable age for Ud-hiyah (sacrificial animal offered by non-pilgrims). Therefore it is impermissible to take older ones from their owners without their consent.