Sahih Bukhari Volume 1, Book 2, Hadith 43

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

Narrated By 'Aisha : Once the Prophet 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."