Yahya related to me from Malik from Yunus ibn Yusuf from Said ibnal-Musayyab that Umar ibn al-Khattab passed by Hatab ibn Abi Baltaawho was underselling some of his raisins in the market. Umar ibn al-Khattab said to him, "Either increase the price or leave our market."
USC-MSA web (English) reference: Book 31, Hadith 57
Arabic reference: Book 31, Hadith 1349