How authentic is the Hadith in which Allah's Messenger (peace be upon him) said what means, "Whoever comes to the Masjid (mosque) after the Adhan (call to Prayer) has been proclaimed is a bad person"?
The Glorious Qur'an urges on hastening to do good deeds, making it a competition for seeking Allah's Forgiveness and Mercy. Such a competition includes offering the Five Obligatory Daily Prayers in the Masjid at their prescribed times, promising a great reward for that. However, this does not denote that whoever comes to the Masjid after the Adhan has been proclaimed is a bad person. On the contrary, that person may be one of the most pious Muslims. A bad person (Part No. 6; Page No. 330) is one who delays the Salah (Prayer) beyond its prescribed time or does not observe it in congregation.
Ibn `Umar (may Allah be pleased with them) reported that the Prophet (peace be upon him) said:
There are three types of people whose Salah is not accepted by Allah: One who leads people in Salah while they do not like him, a person who is always late to Salah, and a person who takes into slavery an emancipated (male or female) slave.
(Related by Abu Dawud and Ibn Majah).
As for the Hadith you have mentioned at the beginning of your question, I know no origin for it.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.