What is the ruling on praying behind an Imam who combines Dhuhr with Asr and Maghrib with Isha several times a month without a general excuse like rain or cold, claiming that he has a private hardship that he is not obliged to inform the worshippers about, knowing that he is seen outside the mosque at the time of the second prayer?

It is permissible for a sick Imam to combine two prayers while at home if he experiences hardship, based on the hadith (definition) of Ibn Abbas narrated by Muslim. However, it is not permissible for healthy congregants to combine prayers with him; rather, they should only perform the current prayer with him. If a congregant sees the Imam outside his home after he has combined the prayers, this does not mean that combining prayers was impermissible for him, as his excuse might be hidden. Nevertheless, the Imam should be advised if it becomes clear that he is taking combining prayers lightly.