Sahih Bukhari
Fighting for the Cause of Allah (Jihaad)
Volume: 004 / Book Number: 052 / Hadith Number: 246
Narrated 'Abdullah bin 'Umar:
Umar bin Al-Khattab gave a horse to be ridden in Allah's Cause and then he found it being sold. He intended to purchase it. So, he consulted Allah's Apostle who said, "Don't buy it and don't take back your gift of charity."