Sunan An Nasai 2617

أَخْبَرَنَا هَارُونُ بْنُ إِسْحَاقَ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ حَدَّثَنَا عَبْدُ الرَّزَّاقِ، ‏‏‏‏‏‏عَنْ مَعْمَرٍ، ‏‏‏‏‏‏عَنِ الزُّهْرِيِّ، ‏‏‏‏‏‏عَنْ سَالِمِ بْنِ عَبْدِ اللَّهِ، ‏‏‏‏‏‏عَنْ أَبِيهِ، ‏‏‏‏‏‏عَنْ عُمَرَ، ‏‏‏‏‏‏أَنَّهُ حَمَلَ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ فَرَآهَا تُبَاعُ، ‏‏‏‏‏‏فَأَرَادَ شِرَاءَهَا،‏‏‏‏ فَقَالَ لَهُ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ:‏‏‏‏ لَا تَعْرِضْ فِي صَدَقَتِكَ .

It was narrated from 'Umar that: he gave someone a horse to ride in the cause of Allah, then he saw it offered for sale and wanted to buy it. The Prophet said to him: Do not think of buying back what you have given in charity.