Hadith: Leave what you doubt for what you do not doubt; indeed, truthfulness is tranquility, and lying is doubt

عَنْ أَبِي الحَوْرَاءِ السَّعْدِيِّ قَالَ: قُلْتُ لِلْحَسَنِ بْنِ عَلِيٍّ رضي الله عنهما: مَا حَفِظْتَ مِنْ رَسُولِ اللهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ؟ قَالَ: حَفِظْتُ مِنْ رَسُولِ اللهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ:
«دَعْ مَا يَرِيبُكَ إِلَى مَا لاَ يَرِيبُكَ، فَإِنَّ الصِّدْقَ طُمَأْنِينَةٌ، وَإِنَّ الكَذِبَ رِيبَةٌ».
[صحيح] - [رواه الترمذي والنسائي وأحمد] - [سنن الترمذي: 2518]

Abu al-Hawrā’ as-Sa‘di reported: I said to Al-Hasan ibn ‘Ali (may Allah be pleased with him and his father): What did you memorize from the Messenger of Allah (may Allah's peace and blessings be upon him)? He said: I memorized from the Messenger of Allah (may Allah's peace and blessings be upon him):
"Leave what you doubt for what you do not doubt; indeed, truthfulness is tranquility, and lying is doubt."
[Authentic hadith] - - [Sunan At-Termedhy - 2518]

Explanation

The Prophet (may Allah's peace and blessings be upon him) commanded that one should leave whatever words or deeds he has doubts about, whether they are prohibited or not, unlawful or lawful, and choose instead what is not doubtful—things whose goodness and lawfulness he is certain about, as such things bring tranquility and comfort to the heart, whereas whatever is doubtful sows anxiety and confusion in the heart.