كتاب الإيمان
Chapter: Ad-Din (good, righteous deed - act of worship) loved most by Allah Jalla Jalalahu is that which is done regularly. (And in fact the best religion with Allah is Islam)
باب أَحَبُّ الدِّينِ إِلَى اللَّهِ أَدْوَمُهُ
Sahih al-Bukhari 43

Narrated 'Aisha:

Once the Prophet (ﷺ) came while a woman was sitting with me. He said, "Who is she?" I replied, "She is so and so," and told him about her (excessive) praying. He said disapprovingly, "Do (good) deeds which is within your capacity (without being overtaxed) as Allah does not get tired (of giving rewards) but (surely) you will get tired and the best deed (act of Worship) in the sight of Allah is that which is done regularly."

حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا يَحْيَى، عَنْ هِشَامٍ، قَالَ أَخْبَرَنِي أَبِي، عَنْ عَائِشَةَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم دَخَلَ عَلَيْهَا وَعِنْدَهَا امْرَأَةٌ قَالَ ‏"‏ مَنْ هَذِهِ ‏"‏‏.‏ قَالَتْ فُلاَنَةُ‏.‏ تَذْكُرُ مِنْ صَلاَتِهَا‏.‏ قَالَ ‏"‏ مَهْ، عَلَيْكُمْ بِمَا تُطِيقُونَ، فَوَاللَّهِ لاَ يَمَلُّ اللَّهُ حَتَّى تَمَلُّوا ‏"‏‏.‏ وَكَانَ أَحَبَّ الدِّينِ إِلَيْهِ مَا دَامَ عَلَيْهِ صَاحِبُهُ‏.‏
Reference : Sahih al-Bukhari 43
In-book reference : Book 2, Hadith 36
USC-MSA web (English) reference : Vol. 1, Book 2, Hadith 42
  (deprecated numbering scheme)