Allah states that He has perfect knowledge of the good deeds performed by all of His creation, such as charity and various vows, and He rewards tremendously for these deeds, provided they are performed seeking His Face and His promise. Allah also warns those who do not work in his obedience, but instead disobey His command, reject His revelation and worship others besides Him:

وَمَا لِلظَّـلِمِينَ مِنْ أَنصَارٍ

(And for the wrongdoers there are no helpers.) meaning, who will save them from Allah's anger and torment on the Day of Resurrection.

(5) Verse 271: إِن تُبْدُوا الصَّدَقَاتِ فَنِعِمَّا هِيَ (الی قولہ) وَاللَّـهُ بِمَا تَعْمَلُونَ خَبِير : "If you give the alms openly, it is good enough, ... and Allah is All-Aware of what you do"

Obviously, this verse covers all types of charity, whether obligatory or supererogatory, and it is more preferable to perform all of them as secretly as possible. To observe secrecy in a charitable act has religious merits, because it is far from the possibility of riya' ریاء (show off), and more graceful for the receiver who can feel shy about receiving charity in public. It is also beneficial from an earthly point of view, because it does not disclose the amount of wealth one owns.