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It was narrated that Thawban said: "The Messenger of Allah (ﷺ) said: 'Adhere to righteousness even though you will not be able to do all acts of virtue. Know that the best of your deeds is Salat (prayer) and that no one maintains his ablution except a believer.'"
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَلِيُّ بْنُ مُحَمَّدٍ، حَدَّثَنَا وَكِيعٌ، عَنْ سُفْيَانَ، عَنْ مَنْصُورٍ، عَنْ سَالِمِ بْنِ أَبِي الْجَعْدِ، عَنْ ثَوْبَانَ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " اسْتَقِيمُوا وَلَنْ تُحْصُوا وَاعْلَمُوا أَنَّ خَيْرَ أَعْمَالِكُمُ الصَّلاَةُ وَلاَ يُحَافِظُ عَلَى الْوُضُوءِ إِلاَّ مُؤْمِنٌ " .
sunnah.com