Musnad Ahmad 27:1856

'Abd al-Razzaq narrated to us, Sufyan informed us, from Abu Ishaq, from al-Najrani, from Ibn 'Umar, who said: "A man bought palm trees from a man, but they did not produce anything that year. They met and disputed before the Prophet, peace and blessings of Allah be upon him. The Prophet, peace and blessings of Allah be upon him, said: 'By what right do you deem his dirhams lawful? Return his dirhams to him, and do not enter into a forward sale (salam) for palm trees until their ripeness becomes apparent.' I asked Masruq: 'What is their ripeness?' He said: 'That they turn red or yellow.'"