i havn't got the quotes or quotes with me at the mo, but i read/heard [in a nut shell] Sayiddna Adam [as] wanted to witness the nur [light] of Prophet Muhammad pbuh, the same nur which everything is created from, this was the nur that was given by none other than Allah swt.....
So he asked 2 see this Nur of the person who was yet to come but his name existed along side Allah's in the heavens rather than his, after all he was the first human....
So Allah swt answered his request, the nur was going through the back of Sayiddna Adam [as] but he could not see it. so again he requested to see it....now the nur was shining through his forehead, so agan he couldn't see. He asked again, and he was told to put out his hand, and his shahada finger, fore finger?....he did so.....and the nur was reflected on tho his finger, and upon seeing this, he kissed his thumbs and rubbed his eyes...hence you can call it a sunnah of Adam [as]
There is another and i believe it is when hadhrat Bilal heard the adhan he did the same action, and the Prophet mentioned that he liked this action? ....
well people can claim that these hadith's are weak, and others use this as an arguement....
final words...Allahu-alim, Allah knows best.
thats my 2p worth
ma'salama