Sahih Muslim Book - 24, Hadith - 4165

حَدَّثَنِى أُمَيَّةُ بْنُ بِسْطَامَ حَدَّثَنَا يَزِيدُ - يَعْنِى ابْنَ زُرَيْعٍ - حَدَّثَنَا رَوْحٌ - وَهُوَ ابْنُ الْقَاسِمِ - عَنْ زَيْدِ بْنِ أَسْلَمَ عَنْ أَبِيهِ عَنْ عُمَرَ أَنَّهُ حَمَلَ عَلَى فَرَسٍ فِى سَبِيلِ اللَّهِ فَوَجَدَهُ عِنْدَ صَاحِبِهِ وَقَدْ أَضَاعَهُ وَكَانَ قَلِيلَ الْمَالِ فَأَرَادَ أَنْ يَشْتَرِيَهُ فَأَتَى رَسُولَ اللَّهِ -صلى الله عليه وسلم- فَذَكَرَ ذَلِكَ لَهُ فَقَالَ « لاَ تَشْتَرِهِ وَإِنْ أُعْطِيتَهُ بِدِرْهَمٍ فَإِنَّ مَثَلَ الْعَائِدِ فِى صَدَقَتِهِ كَمَثَلِ الْكَلْبِ يَعُودُ فِى قَيْئِهِ ».

It was narrated from 'Umar that he donated a horse (to be ridden in Jihad) in the cause of Allah, and he found it with its owner who had neglected it. He was of poor means and 'Umar wanted to buy it, so he went to the Messenger of Allah (s.a.w) and told him about that. He said: "Do not buy it, even if he gives it to you for a Dirham, for the likeness of the one who takes back his charity is that of the dog who returns to his vomit."