Hakim asked (the Prophet): Messenger of Allah, a man comes to me and wants me to sell him something which is not in my possession. Should I buy it for him from the market? He replied: Do not sell what you do not possess.  

يَا رَسُولَ اللَّهِ يَأْتِينِي الرَّجُلُ فَيُرِيدُ مِنِّي الْبَيْعَ لَيْسَ عِنْدِي أَفَأَبْتَاعُهُ لَهُ مِنَ السُّوقِ فَقَالَ لاَ تَبِعْ مَا لَيْسَ عِنْدَكَ  

On his father's authority, said that his grandfather 'Abd Allah b. 'Amr reported the Messenger of Allah ﷺ as saying: The proviso of a loan combined with a sale is not allowable, nor two conditions relating to one transaction, nor profit arising from something which is not in one's charge, nor selling what is not in your possession.  

قَالَ رَسُولُ اللَّهِ ﷺ لاَ يَحِلُّ سَلَفٌ وَبَيْعٌ وَلاَ شَرْطَانِ فِي بَيْعٍ وَلاَ رِبْحُ مَا لَمْ تَضْمَنْ وَلاَ بَيْعُ مَا لَيْسَ عِنْدَكَ