Musnad Ahmad 166
It was narrated from Zaid bin Aslarn, from his father that ‘Umar gave a horse (to a man for jihad) for the sake of Allah, then he saw it or one of its foals being offered for sale, and he wanted to buy it. He asked the 

Prophet ﷺ about it and he said:
`Leave it, and it (its reward) will come to you or you will have them all (i.e., the reward of all of them).` On one occasion he said: He told him not to (buy it) and he said: 

“Do not buy it and do not take back your charity.`
حَدَّثَنَا سُفْيَانُ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنْ أَبِيهِ، أَنَّ عُمَرَ، حَمَلَ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ فَرَآهَا أَوْ بَعْضَ نِتَاجِهَا يُبَاعُ فَأَرَادَ شِرَاءَهُ فَسَأَلَ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عَنْهُ فَقَالَ اتْرُكْهَا تُوَافِكَ أَوْ تَلْقَهَا جَمِيعًا وَقَالَ مَرَّتَيْنِ فَنَهَاهُ وَقَالَ لَا تَشْتَرِهِ وَلَا تَعُدْ فِي صَدَقَتِكَ‏.‏