Sahih Bukhari Volume 009, Book 092, Hadith Number 396.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 92. Holding Fast to the Qur'an and Sunnah
Narrated By Anas : We were with 'Umar and he said, "We have been forbidden to undertake a difficult task beyond our capability (i.e. to exceed the religious limits e.g., to clean the inside of the eyes while doing ablution)."