"I went out with the Messenger of Allah and heard a man reciting Qul Huwa Allahu Ahad so the Messenger of Allah (ﷺ) said: 'It is obligatory.' I said: 'What is obligatory?' He said: 'Paradise.'"
Jami` at-Tirmidhi Book 45, Hadith 23
← Previous Hadith that the Messenger of Allah (ﷺ) said: "Would one of you like to recite a third of the Qur'an during a night? Whoever recited: Allaahu Al-Wahid As-Samad then he has recited a third of the Qur'an."

Jami` at-Tirmidhi Book 45, Hadith 22 Next Hadith → that the Prophet (ﷺ) said: "Whoever recited Qul Huwa Allahu Ahad two hundred times everyday, fifty years worth of his sins will be removed - unless he owed a debt."

And another narration with this chain, from the Prophet (ﷺ) that he said: "Whoever wants to sleep upon his bed and sleeps on his right side, then he recites Qul Huwa Allahu Ahad one hundred times, then on the Day of Judgement the Lord, Blessed and Most High shall say: 'O My slave! Enter Paradise on your right.'"

Jami` at-Tirmidhi Book 45, Hadith 24