Qurrah bin Ilyaas Radiyallahu'Anhu relates:
"I came with a group from the tribe of Muzeenah to make bay'ah (pay allegiance) to Rasulullah Sallallahu 'Alayhi Wasallam. The button loop of Rasulullah Sallallahu 'Alayhi Wasallam's qamis was open. I put my hand in the collar of Rasulullah Sallallahu 'Alayhi Wasallam's qamis to touch the Seal of Prophethood (to gain barakah)".
English reference: Book 8, Hadith 57
Arabic reference: Book 8, Hadith 58