"O Messenger of Allah! Shall I tie it and rely(upon Allah), or leave it loose and rely(upon Allah)?" He said: "Tie it and rely(upon Allah)." Other chains report similar narrations.
Jami` at-Tirmidhi Book 37, Hadith 103
← Previous Hadith "I was behind the Prophet(s.a.w) one day when he said: 'O boy! I will teach you a statement: Be mindful of Allah and He will protect you. Be mindful of Allah and you will find Him before you. When you ask, ask Allah, and when you seek aid, seek Allah's aid. Know that if the entire creation were to gather together to do something to benefit you- you would never get any benefit except that Allah had written for you. And if they were to gather to do something to harm you- you would never be harmed except that Allah had written for you. The pens are lifted and the pages are dried.'"

Jami` at-Tirmidhi Book 37, Hadith 102 Next Hadith → "I remember that the Messenger of Allah (s.a.w) said: 'Leave what makes you in doubt for what does not make you in doubt. The truth brings tranquility while falsehood sows doubt.'" Another chain reports a similar narration.

Jami` at-Tirmidhi Book 37, Hadith 104