Sahih Muslim
The Book Pertaining to Paradise, Its Description, Its Bounties and Its Intimates (Kitab Al-Jannat wa Sifat Naimiha wa Ahliha)
Book Number: 040 / Hadith Number: 6850
This hadith has been transmitted on the authority of Ibn 'Umar but with a slight variation of wording (and the words are):" One of them would be completely submerged in perspiration up to half of his ears."