صحيح البخاري

Narrated Abu Dhar:

I asked the Prophet, "What is the best deed?" He replied, "To believe in Allah and to fight for His Cause." I then asked, "What is the best kind of manumission (of slaves)?" He replied, "The manumission of the most expensive slave and the most beloved by his master." I said, "If I cannot afford to do that?" He said, "Help the weak or do good for a person who cannot work for himself." I said, "If I cannot do that?" He said, "Refrain from harming others for this will be regarded as a charitable deed for your own good."