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