Musnad Ahmad 26:129

Muhammad ibn al-Sammak narrated to us, from Yazid ibn Abi Ziyad, from al-Musayyab ibn Rafi’, from ‘Abdullah ibn Mas’ud, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not buy fish while it is in the water, for it is uncertainty (gharar)."