Sahih al-Bukhari 7293
Sahih al-Bukhari · Book 96: Holding Fast to the Qur'an and Sunnah · Asking too many questions and troubling with what does not concern one
Narrated Anas:
حَدَّثَنَا سُلَيْمَانُ بْنُ حَرْبٍ ، حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ ، عَنْ ثَابِتٍ ، عَنْ أَنَسٍ ، قَالَ كُنَّا عِنْدَ عُمَرَ فَقَالَ نُهِينَا عَنِ التَّكَلُّفِ.
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).