Is one who dares not advise another who errs in prayer sinful, given that he is by nature shy and cannot overcome his shyness except with those close to him?

Whoever catches the Imam while he is bowing (ruku'), it suffices him to say one takbiratul-ihram (definition) while standing, and then he bows. However, if he also says takbir for bowing, it is better. Enjoining good and forbidding evil are among the confirmed rites of religion and should not be abandoned due to shyness or embarrassment. One must strive against himself in this regard until the blameworthy shyness is removed from him. The condition for this is to be certain of the correctness of what he calls to, and to adhere to the etiquette of enjoining good and forbidding evil, such as good manners, gentleness, and patience in the face of harm.