'Aayeshah Radiyallahu 'Anha reports:
(because of the excessive modesty of Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam) "I did not have the courage to see the private parts of Rasulullah Sallallahu'Alayhi Wasallam, or that I never saw the private parts of Rasulullah Sallallahu 'Alayhi Wasallam". (ID)
English reference (ID): Book 48, Hadith 342 (ID)
Arabic reference (ID): Book 49, Hadith 359 (ID)