Narrated Aslam: I heard `Umar bin Al-Khattab saying, "I gave a horse to be ridden in Allah's Cause and the person who got it intended to sell it or neglected it. So, I wanted to buy it as I thought he would sell it cheap. I consulted the Prophet (ﷺ) who said, "Do not buy it even if for one Dirham, because he who takes back his gift is like a dog swallowing its vomit."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا إِسْمَاعِيلُ، حَدَّثَنِي مَالِكٌ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنْ أَبِيهِ، قَالَ سَمِعْتُ عُمَرَ بْنَ الْخَطَّابِ ـ رضى الله عنه ـ يَقُولُ حَمَلْتُ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ فَابْتَاعَهُ ـ أَوْ فَأَضَاعَهُ ـ الَّذِي كَانَ عِنْدَهُ، فَأَرَدْتُ أَنْ أَشْتَرِيَهُ، وَظَنَنْتُ أَنَّهُ بَائِعُهُ بِرُخْصٍ، فَسَأَلْتُ النَّبِيَّ صلى الله عليه وسلم فَقَالَ " لاَ تَشْتَرِهِ وَإِنْ بِدِرْهَمٍ، فَإِنَّ الْعَائِدَ فِي هِبَتِهِ كَالْكَلْبِ يَعُودُ فِي قَيْئِهِ ".
sunnah.com