Sahih Bukhari Volume 2, Book 24, Hadith 1490

حَدَّثَنَا يَحْيَى بْنُ بُكَيْرٍ، حَدَّثَنَا اللَّيْثُ، عَنْ عُقَيْلٍ، عَنِ ابْنِ شِهَابٍ، عَنْ سَالِمٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ ـ رضى الله عنهما ـ كَانَ يُحَدِّثُ أَنَّ عُمَرَ بْنَ الْخَطَّابِ تَصَدَّقَ بِفَرَسٍ فِي سَبِيلِ اللَّهِ فَوَجَدَهُ يُبَاعُ، فَأَرَادَ أَنْ يَشْتَرِيَهُ، ثُمَّ أَتَى النَّبِيَّ صلى الله عليه وسلم فَاسْتَأْمَرَهُ فَقَالَ ‏"‏ لاَ تَعُدْ فِي صَدَقَتِكَ ‏"‏ فَبِذَلِكَ كَانَ ابْنُ عُمَرَ ـ رضى الله عنهما ـ لاَ يَتْرُكُ أَنْ يَبْتَاعَ شَيْئًا تَصَدَّقَ بِهِ إِلاَّ جَعَلَهُ صَدَقَةً‏

Narrated By 'Abdullah bin 'Umar : Umar bin Al-Khattab gave a horse in charity in Allah's Cause and later he saw it being sold in the market and intended to purchase it. Then he went to the Prophet and asked his permission. The Prophet said, "Do not take back what you have given in charity." For this reason, Ibn 'Umar never purchased the things which he had given in charity, and in case he had purchased something (unknowingly) he would give it in charity again.