Sahih al-Bukhari 7293

Arabic

حَدَّثَنَا سُلَيْمَانُ بْنُ حَرْبٍ، حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، عَنْ ثَابِتٍ، عَنْ أَنَسٍ، قَالَ كُنَّا عِنْدَ عُمَرَ فَقَالَ نُهِينَا عَنِ التَّكَلُّفِ‏.‏

English translation

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)

Reference: Sahih al-Bukhari 7293, Book of Holding Fast to the Qur'an and Sunnah. English translation by Dr. Muhammad Muhsin Khan.