Taking as evidence what we have narrated in the Book of Sales from the Prophet (peace and blessings be upon him), that he (peace and blessings be upon him) forbade the sale involving deception (gharar); and hire contracts are a type of sale, and the lack of knowledge (of anything) in them is deception.
It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "A man should not bargain over his brother's bargain, nor propose over his brother's proposal, nor bid (to inflate the price), nor transact sales by throwing a stone; and whoever hires someone should settle his wage with him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإجارة / 11651