Translation
(96) Holding Fast to the Qur'an and Sunnah
كتاب الاعتصام بالكتاب والسنة
(3) Chapter: Asking too many questions and troubling with what does not concern one
(3) 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)