Majma al-Zawaid — Hadith 18116

Sayyiduna Wathilah (may Allah be pleased with him) narrates: I said: "O Prophet of Allah! Tell me." The Prophet (peace and blessings be upon him) said: "If you wish, I will tell you about the thing you have come to ask about, and if you wish, you may ask." I said: "Rather, tell me, O Messenger of Allah! For that will be more reassuring for me." The Prophet (peace and blessings be upon him) said: "You have come to ask about certainty and doubt." I said: "That is so." The Prophet (peace and blessings be upon him) said: "Certainty is what settles firmly in the chest and with which the heart is at peace, whatever rulings those who give rulings may give you. Leave what puts you in doubt and go to what does not put you in doubt, for goodness is tranquillity and doubt is unease. When you are in doubt, leave it." After that the narrator mentioned the hadith as before.
Al-Tabarani recorded it; its chain contains a narrator, Isma'il bin Abdullah al-Kindi, who is da'if (weak).