Sayyiduna Abu Ayyub (may Allah be pleased with him) said that he heard the Prophet (peace and blessings be upon him) say: "Soon cities will be conquered for you, and there will be assembled armies in which expeditions will be assigned to you (by turns). In that time a man will dislike going out in them and will slip away from his people; then he will look over the tribes, offering himself to them and saying: 'Whom shall I replace in such-and-such expedition? Whom shall I stand in for in such-and-such expedition?' Beware! He is a hired worker (one who fights for wages) to the last drop of his blood."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17837
Shaqiq bin al-Ayzar said: I asked Ibn Umar (may Allah be pleased with them both) about payment (for going out to fight), and he said: I do not take any payment except the payment my Lord gives me. Shaqiq said: I asked Abdullah bin al-Zubayr (may Allah be pleased with them both), and he said: Leaving it is better; if you do take it, then spend it in the way of Allah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17838
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
Jubayr bin Nufayr (may Allah have mercy on him) said that the Prophet (peace and blessings be upon him) said: "The example of those of my Ummah who take payment for fighting and use it to gain strength against the enemy is like the mother of Musa (peace be upon him): she nursed her son and took a wage for it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17840