Musannaf Ibn Abi Shayba 13:2218

Abu Bakr narrated to us, he said: Ibn Mubarak narrated to us, from Ma'mar, from al-Zuhri, that the Prophet (may Allah bless him and grant him peace) passed by a Bedouin who was selling something. He said: "You must take the first offer (or the first bargaining), for indeed, profits are with generosity."