Abu Dharr (may Allah be pleased with him) reported: 'It was said to the Messenger of Allah (may Allah’s peace and blessings be upon him): "What do you think of a man who does a good deed and people praise him for it?" He said: "That is instant good news for the believer."
Here we are told of a man who does a righteous deed for the sake of Allah, without intending it for others. People, however, commend him and say that he is a charitable person, etc. The Prophet (may Allah's peace and blessings be upon him) says that such an appreciation is an instant good news for the believer, because people are witnesses over Allah's creation, so their testimony does count. That is why when a coffin passed by the Prophet (may Allah's peace and blessings be upon him) and his Companions spoke well of the deceased, he said that they confirmed that he would be admitted to Paradise. Also, when another coffin passed by him and the Companions spoke badly of the deceased, the Prophet (may Allah's peace and blessings be upon him) said that they confirmed that he would be admitted to Hell. In both cases, he justified that people are Allah's witnesses on earth. So, the commendation that a believer gets from people for the good thing that he does is an instant good news for him in the world. However, the one who does something ostentatiously to draw people's attention and favorable remarks, has thus associated partners with Allah. This is what makes the other sincere person greatly different from him.