Kissing the thumbs during the Azan
In the name of Allah, the most Beneficent, the most Merciful.
It is narrated that whenever Saaiduna Abu Bakr Radiallahu Anhu heard the Muazin say: "I bear witness that Muhammad is the Messenger of Allah", he would repeat this phrase and would kiss the tip of the index fingers [or thumbs] and wipe his eyes. The Prophet of Allah Sallallahu Alahi Wasalam said: ‘whosoever does what my friend i.e., Abu Bakr did, my intercession will come down upon him’. (Sunan Dailami)
The aforementioned hadith is weak. However, weak narrations are not acceptable only in regards to aqeedah and in establishing Shariah rulings.
Scholars such as Imam Ibn Salah and Imam Nawawi have always stated that there is a leeway in quoting weak hadiths in acts of virtue. Even Imam Bukhari (RA) brought weak narrations in his book “Al Adabul Mufrad”, indicating that he also accepts weak narrations in regards to virtues of good deeds.
With respect to your question, there is nothing wrong in quoting weak hadiths in regards to virtues of good deeds. This is an Islamic principle therefore if one wants to kiss his thumbs at the time of Azan and wipe it over his eyes then there is nothing wrong with it.
Mufti Taqi Uthmani (DB) has said that if someone kissed his thumb during the azan out of love of the Prophet Sallallahu Alahi Wasalam then this action cannot be considered sinful or bidah. The reason being is that one did this out of love for the Prophet and love and respect for the prophet is a praiseworthy act and a sign of faith. (Islahi Khutubat p.231 v.1)
However, what is wrong is that some people deem the aforementioned act as something necessary. To deem a praiseworthy act as something necessary then becomes a bidah in Islam.
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham.