Mishkat al-Masabih 2773
Al-Hasan b. ‘Ali said:
I have retained in my memory these words of God's Messenger, “Leave what causes you doubt and turn to what does not cause you doubt. Truth is tranquility, but falsehood is doubt.” 

Ahmad, Tirmidhi and Nasa’i transmitted it, and Darimi transmitted the first part.
وَعَنِ الْحَسَنِ بْنِ عَلِيٍّ رَضِيَ اللَّهُ عَنْهُمَا قَالَ: حَفِظْتُ مِنْ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «دَعْ مَا يَرِيبُكَ إِلَى مَا لَا يَرِيبُكَ فَإِنَّ الصِّدْقَ طُمَأْنِينَةٌ وَإِنَّ الْكَذِبَ رِيبَةٌ» . رَوَاهُ أَحْمَدُ وَالتِّرْمِذِيُّ وَالنَّسَائِيُّ وَرَوَى الدَّارِمِيُّ الْفَصْل الأول