Reference: Fataawa al-Lajnah ad-Daa.imah – Question No.3 of Fatwa No. 6756
al-Bid’u wal-Muhdathaat wa maa laa asla lahu – Page 191
Question: Is there any basis for kissing the-thumbs when the mu.adhdhin says أشهد أن محمداً رسول الله ?
Response: From what we know, [the act of] kissing the thumbs when the mu.adhdhin says أشهد أن محمداً رسول الله has not been confirmed on the authority of the Prophet ﷺ. So kissing them then is an innovation, and it has been confirmed on the authority of the Prophet ﷺ that he said:
«Whoever introduces anything into this matter of ours (i.e. Islaam) that which is not from it, will have it rejected», [Saheeh al-Bukhaaree/2697 and Saheeh Muslim/1718].
And with Allaah lies all success and may Allaah send prayers and salutations upon our Prophet ﷺ and his family and his companions.