Is it permissible to supplicate during a silent prayer when one is praying behind the Imam?

It is permissible for the congregant to supplicate in his prayer behind the Imam, whether the prayer is silent or audible. This should be done in places of supplication such as prostration (sujud) and sitting between the two prostrations. As for supplication while standing, it is not permissible, but rather disliked. What is prescribed during standing is the recitation of the Qur'an, unless a verse of mercy is recited, in which case asking for it is permissible, or a verse of punishment, in which case seeking refuge is permissible.