Sahih Muslim
The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness (Kitab Al-Dhikr)
Book Number: 035 / Hadith Number: 6506
Qatada asked Anas which Supplication Allah's Apostle (may peace be upon him) frequently made. He said: The supplication that he (the Holy Prophet made very frequently is this:" O Allah, grant us the good in this world and the good in the Hereafter and save us from the torment of Hell-Fire." He (Qatada) said that whenever Anas had to supplicate he made this very supplication, and whenever he (intended) to make another supplication he (inserted) this very supplication in that.