What is the ruling on searching for a specific person to pray next to him in the mosque?

The basic principle is that there is no harm in praying near anyone, unless it prevents one from praying in the front rows. The most important thing is to focus on praying in the places that the Prophet, peace and blessings be upon him, encouraged hastening towards, and not to be concerned with praying next to a specific person. If the desire to pray next to a specific person stems from love for the sake of Allah, then this has great merit, provided there is no extremism or negligence. However, if it is for an illegitimate purpose, then one must avoid everything that leads to what Allah and His Messenger disapprove of, as a preventative measure.