Because Allah the Exalted has said that those who spend their wealth in the way of Allah and do not follow up what they have spent with reminders of their generosity or with harm.
Narrated `Uqba bin Al-Harith: Once the Prophet offered the `Asr prayer and then hurriedly went to his house and returned immediately. I (or somebody else) asked him (as to what was the matter) and he said, "I left at home a piece of gold which was from the charity and I disliked to let it remain a night in my house, so I got it distributed . "
Hadith Referenceصحيح البخاري / كتاب الزكاة / 1430
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة