It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) "forbade hiring a worker, that is, until he makes his wage clear to him."
It is mursal (with a break in the chain) between Ibrahim and Abu Sa'id.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإجارة / 11652