Musannaf Ibn Abi Shayba 13:2352

Abu Bakr narrated to us, he said: Ibn Numayr narrated to us, from Ubayd Allah, from Nafi’, from Ibn Umar: That he did not see any harm in a man paying in advance for food with a known measure to a known term, as long as it was not in crops or dates before their ripeness became apparent.