ENOfficial
Narrated Hakim ibn Hizam: 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.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ أَبِي بِشْرٍ، عَنْ يُوسُفَ بْنِ مَاهَكَ، عَنْ حَكِيمِ بْنِ حِزَامٍ، قَالَ يَا رَسُولَ اللَّهِ يَأْتِينِي الرَّجُلُ فَيُرِيدُ مِنِّي الْبَيْعَ لَيْسَ عِنْدِي أَفَأَبْتَاعُهُ لَهُ مِنَ السُّوقِ فَقَالَ " لاَ تَبِعْ مَا لَيْسَ عِنْدَكَ " .
sunnah.com