Musannaf Ibn Abi Shayba 13:1086

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: I witnessed before Ibn Abi Layla some boys from the neighborhood who had not reached puberty. He said: "Write: 'So-and-so and so-and-so witnessed,' and they are young and have not reached puberty. Then, if they reach puberty and remain firm upon their testimony, it is valid. And if they retract it, then it is nothing."