Sahih Bukhari Volume 9, Book 92, Hadith 7058

فَقَالَ فِيمَا أَخَذَ عَلَيْنَا أَنْ بَايَعَنَا عَلَى السَّمْعِ وَالطَّاعَةِ، فِي مَنْشَطِنَا وَمَكْرَهِنَا، وَعُسْرِنَا، وَيُسْرِنَا، وَأَثَرَةٍ عَلَيْنَا، وَأَنْ لاَ نُنَازِعَ الأَمْرَ أَهْلَهُ، إِلاَّ أَنْ تَرَوْا كُفْرًا بَوَاحًا، عِنْدَكُمْ مِنَ اللَّهِ فِيهِ بُرْهَانٌ‏.‏

and among the conditions on which he took the Pledge from us, was that we were to listen and obey (the orders) both at the time when we were active and at the time when we were tired, and at our difficult time and at our ease and to be obedient to the ruler and give him his right even if he did not give us our right, and not to fight against him unless we noticed him having open Kufr (disbelief) for which we would have a proof with us from Allah."