It is narrated from Sayyiduna Arib Abu Abdullah al-Mulayki (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said that this verse, "Those who spend their wealth by night and by day, secretly and openly" [Surah al-Baqarah: 274], was revealed concerning spending on horses.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14576