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Narrated Hakim b. Hizam: "I asked Messenger of Allah (ﷺ), I said: 'A man came to me asking to buy something that I did not have. Can I buy it from the market for him and then give it to him?' He said: 'Do not sell what is not with you.'" He said: There are narrations on this topic from 'Abdullah bin 'Umar.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا قُتَيْبَةُ، حَدَّثَنَا هُشَيْمٌ، عَنْ أَبِي بِشْرٍ، عَنْ يُوسُفَ بْنِ مَاهَكَ، عَنْ حَكِيمِ بْنِ حِزَامٍ، قَالَ أَتَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم فَقُلْتُ يَأْتِينِي الرَّجُلُ يَسْأَلُنِي مِنَ الْبَيْعِ مَا لَيْسَ عِنْدِي أَبْتَاعُ لَهُ مِنَ السُّوقِ ثُمَّ أَبِيعُهُ قَالَ " لاَ تَبِعْ مَا لَيْسَ عِنْدَكَ " . قَالَ وَفِي الْبَابِ عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ .
sunnah.com