Musannaf Ibn Abi Shayba 13:241

Isma'il ibn 'Ayyash narrated to us, from 'Amr ibn Muhajir, that 'Umar ibn 'Abd al-'Aziz sent 'Amrah ibn Zayd al-Filastini to sell captives to whoever offered more. When he finished, he came to him, and 'Umar said to him, "How was the sale today?" He said, "The sale was sluggish, O Commander of the Faithful, were it not for the fact that I was bidding against them to get it sold." 'Umar said, "You were bidding against them without intending to buy?" He said, "Yes." 'Umar said, "This is *najash* (bidding up the price without the intention to buy), and *najash* is not lawful. Send a crier, O 'Amrah, to proclaim: 'Indeed, the sale is void, and *najash* is not lawful.'"