'Aayeshah Radiyallahu 'Anha says:
"Rasulullah Sallallahu 'Alayhi Wasallam once came home, a woman was present at that time. Rasulullah Sallallahu 'Alayhi Wasallam inquired. 'Who is this woman?' I replied: 'This is a certain woman who does not sleep at night'. Rasulullah Sallallahu 'Alayhi Wasallam said: 'One should observe only that amount of nawaafil which one can bear. I swear an oath that Allah Ta'aala does not fear the granting of rewards, till you begin to fear the observance of deeds'. 'Aayeshah Radiyallahu. 'Anha says: 'The most beloved deed of Rasulullah Sallallahu 'Alayhi Wasallam were those that were practised continuously.
حَدَّثَنَا هَارُونُ بْنُ إِسْحَاقَ، قَالَ‏:‏ حَدَّثَنَا عَبْدَةُ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، قَالَتْ‏:‏ دَخَلَ عَلَيَّ رَسُولُ اللهِ صلى الله عليه وسلم وَعِنْدِي امْرَأَةٌ، فَقَالَ‏:‏ مَنْ هَذِهِ‏؟‏ قُلْتُ‏:‏ فُلانَةُ لا تَنَامُ اللَّيْلَ، فَقَالَ رَسُولُ اللهِ صلى الله عليه وسلم‏:‏ عَلَيْكُمْ مِنَ الأَعْمَالِ مَا تُطِيقُونَ، فَوَاللَّهِ لا يَمَلُّ اللَّهُ حَتَّى تَمَلُّوا، وَكَانَ أَحَبَّ ذَلِكَ إِلَى رَسُولِ اللهِ صلى الله عليه وسلم الَّذِي يَدُومُ عَلَيْهِ صَاحِبُهُ‏.‏
English reference : Book 42, Hadith 294
Arabic reference : Book 43, Hadith 311