Hadith #2946 (Jami' al-Tirmidhi)
Book: Chapters on Recitation
Narrator(s): Ahmed bin Muhammad bin Nyzk أحمد بن محمد بن نيزك, al-'Aswad bin 'Amir Shadhan الأسود بن عامر شاذان, al-Laith bin Sa'd الليث بن سعد, ibn Umar, Nafie' نافع مولى ابن عمر

that the Prophet (ﷺ) said: “Recite the Qur’an in forty (days).”

Source: https://parallelquran.com/hadith/jami-al-tirmidhi-2946/