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