Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.

 

The hadith is reported in Al-Bukhari. Sahl ibn Sa’d (May Allah be pleased with him) reported: The Messenger of Allah (peace be upon him) said, “Whosoever gives me a guarantee to safeguard what is between his jaws and what is between his legs, I shall guarantee him Jannah.”

 

You do not have to pay kaffarh in either times except if you made a vow. In case you vowed that you  have to make a kaffarh for the first time only. You need to increase your iman. I suggest for you to train yourself on catching up the first row in congregational prayers at masjid for at least a month. I am sure you will not approach this sin again.

 

Almighty Allah knows best.