Allah placed the Nūr (Divine Light) of Prophet Muhammad (peace and blessings be upon him) on the forehead of Hazrat Adam (peace be upon him).

After the creation of Hazrat Adam (A.S.), Mother Hawwa (Eve) was created from him. Adam’s body was first shaped and left to dry for forty days. Afterward, the soul was breathed into Adam (A.S.) (nafakhtu fihi min rūhī — "I breathed into him of My spirit").

Initially, when the soul witnessed darkness within Adam’s body, it refused to enter. Then Allah placed the Nūr-e-Muhammadi (Light of Muhammad) on Adam's forehead, and the darkness of the body was dispelled. At that moment, Adam assumed the form of a human being, sneezed, and recited “Alhamdulillah” (All praise be to Allah). In response, Allah said, “Yarhamukallah” (May Allah have mercy on you).

Following this, the angels were commanded to prostrate before Adam, and all obeyed except Azazil (later known as Iblis or Satan), who refused and became a disbeliever and a cursed one.
This prostration was, in fact, directed to Allah, not to Adam as separate from Him. Since Adam was established in the state of being eternal and deathless (la mawt), prostrating towards Adam was essentially a prostration to Allah Himself.

Thus, it was ordained through the angels that future humanity must recognize and prostrate to Adam (meaning, the Perfect Human), and those who do so will be the saved ones. Those who refuse will belong to the accursed group of Satan.
However, to properly prostrate to Adam, one must recognize him and accept him as Allah’s Caliph (Vicegerent) on earth. Without recognizing Adam, one cannot truly recognize Allah either.

Those who worship a concept of Allah apart from Adam (i.e., apart from the Perfect Human Reality) fall into shirk (associating partners with Allah), because Adam is established with the Essence (Sifat) and Form (Surat) of Allah. The Perfect Human (al-Insan al-Kamil) is Allah’s hidden secret (al-insanu sirri), and thus, to prostrate to Adam is, in essence, to prostrate to Allah.

In the famous book Mawlid Barzanji, Allama Sharif Barzanji (may Allah have mercy on him) wrote:
When Allah intended to manifest the Haqqiqah Muhammadiyyah (Essential Reality of Muhammad), He created Hazrat Adam (A.S.) and placed the Nūr of Prophet Muhammad (peace be upon him) on his forehead.

In Ahsanul Mawaa'iz, it is narrated that one day Hazrat Adam (A.S.) prayed to Allah to be able to see the Nūr-e-Muhammadi. Allah then manifested the Nūr-e-Muhammadi in Adam’s two thumbs like a clear mirror. Seeing this, Adam (A.S.) kissed the nails of his thumbs and touched them to his eyes. Recognizing the Prophet, Adam brought faith (Iman) upon him.

This practice was then passed down to his descendants as a Sunnah.
This incident is also mentioned in Tafsir Ruh al-Bayan.
Thus, kissing the thumbs after hearing the Adhan (call to prayer) or upon hearing the name of the Prophet (peace and blessings be upon him) is a Sunnah.
Whoever observes this Sunnah, Allah will forgive all their sins.

Submitted by:
Shah Sufi Ahmad Hossain (Pagla of Spiritual Sanctity - "Belayet'er Pagla")