Musannaf Ibn Abi Shayba 13:522

Abu Bakr narrated to us, he said: Ibn Abi Za’idah narrated to us, from Yahya ibn Sa’id, from Nafi’, who said: When Ibn Umar was asked about a man who buys something from a man for a deferred term, while he does not possess its principal (the item), he did not see any harm in it. Yahya said: And Sa’id ibn al-Musayyib used to dislike it.