In the name of Allah the Most Beneficent and Merciful
Kissing the thumbs when hearing the name of the Prophet is a Mustahhab act which is backed by weak Hadith. However it is not something you have to do nor are you accountable for it. If done then Insha Allah you will be rewarded. However, Shaykh Imam Ahmad Raza Khan in his Fatawa-e-Radhwiyya has stated three occasions when it should not be done:
1. When hearing the name of the Prophet through the recitation of the Quran (Maa Kaana Muhammadun…..) as to listen to the Quran is Fardh at this time.
2. When the Azan for the Khutba Sermon is called and the Imam is on the Mimbar as to remain quiet and listen is Wajib. (the Imam can kiss his thumbs if he so wishes).
3. Whilst praying Salaah.
Of course Allah knows best
[Answer provided by: Muhammad Salim Ghisa]