The source gives two related rules:
• Leaving the mosque after Adhan before performing the prayer is disliked unless the person is going to establish or attend another valid congregation or has a comparable excuse.
• If someone already prayed alone and a congregation is established before leaving, the person joins it as Nafl in Zuhr and Isha. The source does not extend this post-Fard voluntary repetition to every prayer time.
It also cautions against repeatedly performing the same prayer merely to gain another reward or because of an imagined defect without a valid reason.