Sahih al-Bukhari 7016
In-book no 7016حَدَّثَنَا سُلَيْمَانُ بْنُ حَرْبٍ، حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، عَنْ ثَابِتٍ، عَنْ أَنَسٍ، قَالَ كُنَّا عِنْدَ عُمَرَ فَقَالَ نُهِينَا عَنِ التَّكَلُّفِ.
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).