Sahih Bukhari
To make the Heart Tender (Ar-Riqaq)
Volume: 008 / Book Number: 076 / Hadith Number: 566
Narrated An-Nu'man:
I heard the Prophet saying, "The person who will have the least punishment from amongst the Hell Fire people on the Day of Resurrection, will be a man under whose arch of the feet a smoldering ember will be placed so that his brain will boil because of it."