Musannaf Ibn Abi Shayba 13:876

Abu Bakr narrated to us, he said: Waki' narrated to us, from Sadaqah ibn Musa al-Dimashqi, from al-Wadin ibn 'Ata, who said: There were three teachers in Medina who used to teach the children. Umar ibn al-Khattab used to provide each one of them with fifteen [dirhams] every month.