Maliks Muwatta Book 9, Hadith Number 95.
Section : Prayer in General.
Yahya related to me from Malik that he had heard that if some one passed by Ata ibn Yasar in the mosque with something to trade, he would call him and ask, “What is the matter with you? What do you want?” If the man said that he wished to trade with him, he would say, “You need the market of this world. This is the market of the next world.”