Sahih al-Bukhari 7293
Narrated 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).
حَدَّثَنَا سُلَيْمَانُ بْنُ حَرْبٍ، حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، عَنْ ثَابِتٍ، عَنْ أَنَسٍ، قَالَ كُنَّا عِنْدَ عُمَرَ فَقَالَ نُهِينَا عَنِ التَّكَلُّفِ‏.‏