Should I be sincere in my intention to please Allah when doing good deeds or not, especially knowing that Allah knows best what is in my heart?

A Muslim must intend their actions for the sake of Allah and beware of showing off (riya') or seeking worldly gain through their deeds. For Allah accepts only those deeds that are purely for His sake. He, the Almighty, said: "I am the most self-sufficient partner, aloof from partnership. Whoever performs an action in which he associates another with Me, I will abandon him and his partnership."

A Muslim must not open a door for Satan to cast doubt on their intention, which could lead to abandoning good deeds. Rather, they must purify their intention for Allah, strive against their soul from turning to anyone other than Him, and pay no attention to Satan's whispers.