The noble Shaykh Abdur Razzaaq Ibn Abdil-Muhsin al-Badr (may Allaah preserve them both) stated:

“It is befitting that we make note that the Muslim has no way of being sure that his actions are accepted no matter how well or good he has done in performing the action. Allaah has mentioned, clarifying the state and affair of the complete believers during the performance of actions by which they seek nearness to Allaah from the acts of obedience:

“And those who give that which they give with their hearts full of fear  because they are sure they are to return to their Lord.” (Soorah Mu-minoon: 60)

This means that they give from themselves that which they are commanded with from the acts of worship from Salaah, Zakaah, Hajj, Siyaam (Fasting) and other than that. They are in fear that their actions will not save them or that their acts of worship will not be accepted when their actions are being presented to Allaah and they have to stand in front of Allaah. Imaam Ahmad mentions in his Musnad on the authority of The Mother of The Believers Aaishah (may Allaah be pleased with her) stated:

“I said: Oh Messenger of Allaah (sallallaahu alayhi wa sallam) Allaah has said:

“And those who give that which they give with their hearts full of fear  because they are sure they are to return to their Lord.”  (Soorah Mu-minoon: 60)

Is it the man who commits Zinaa (fornication/adultery) and consumes intoxicants? He (sallallaahu alayhi wa sallam) said: No oh daughter of Abee Bakr/ No oh daughter of as-Siddeeq. However he is the man who fast, prays and gives charity and yet he fears that it wont be accepted from him.”

Taken from Duroos Aqeedah Mustafaadah Min Hajj by Shaykh Abdur Razzaaq Ibn Abdil-Muhsin al-Badr (may Allaah preserve them both) pg. 114, Translated by Abu Yusuf Khaleefah Brooklyn NYC
7th Night of Dhul-Hijjah 1432/ November 2nd 2011

Source : Abu Yusuf Khaleefah’s Site
http://31097.multiply.com/journal/item/90/Dont_Be_Sure_That_Your_Actions_Are_Accepted