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.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإجارة / Q11651