Musannaf Ibn Abi Shayba 11:226

Waki’ narrated to us, from Sufyan, from Abu Ishaq, from Ubayd ibn al-A’jam, who said: I asked Ibn Abbas about the *ja’a’il* (hiring fees/stipends for military service). He said: "If you spend it on weapons or horses in the way of Allah, then there is no harm." He said: "And if you spend it on a male slave or a female slave, then it is of no benefit."