Kissing the thumb of our hand while listening to the name of our Beloved Prophet SAW

Assalam o alaikum,
Sir I want to know whether kissing the thumb of our hand while listening to the name of our Beloved Prophet (Peace be upon him) is Sunnat or bidd’ah? Please quote reference.
Another person gave me this answer:

It is sunnat of the first Khaleefa abu bakar siddiq
Q2. If this is so then can this be regarded as Sunnat as our first Khalifa did this and can we do the same or not. Or will it be still sin. Please avoid confusion.

Jazzak Allah!