Musannaf Ibn Abi Shaybah

It is narrated from Hazrat Zuhri that Hazrat Umar (may Allah be pleased with him) gave a verdict regarding a person who had gifted an animal to another person, and that animal gave birth to a young one. He (Hazrat Umar) said that he should take back its price, according to the value on the day it was gifted.

Hazrat Umar bin Abdul Aziz wrote: He will revoke the gift based on the value of the day it was given, and further wrote that if the gifted item increases (for example, gives birth to a child, etc.), then that increase belongs to the recipient of the gift.