Musnad Ahmad 32:198

Yahya narrated to us, from Humayd, who said: Anas was asked about the selling of fruit, and he said: "The Messenger of Allah, may Allah bless him and grant him peace, forbade the selling of the fruit of the palm tree until it shows color." It was said to Anas: "What is 'shows color' (tazhu)?" He said: "It turns red."