Ubayd bin al-A'jam said that a man asked Ibn Abbas (may Allah be pleased with them both) about payment (for fighting), and he said: When it is for weapons and horses, there is no harm in it; but when the payment is for slaves, then do not take it.
Ibrahim al-Nakha'i said: Giving (such payment) was more beloved to them than taking it, meaning in the matter of payments.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17839