كتاب الاعتصام بالكتاب والسنة
Holding Fast to the Qur'an and Sunnah
Chapter: Asking too many questions and troubling with what does not concern one
باب مَا يُكْرَهُ مِنْ كَثْرَةِ السُّؤَالِ وَتَكَلُّفِ مَا لاَ يَعْنِيهِ
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.‏
Reference : Sahih al-Bukhari 7293
In-book reference : Book 96, Hadith 24
USC-MSA web (English) reference : Vol. 9, Book 92, Hadith 396
  (deprecated numbering scheme)