Abu Dhar (may Allah be pleased with him) narrated that the Messenger of Allah (peace be upon him) said: “Whoever, after Fajr prayer, while remaining seated in the posture of prayer and before speaking to anyone, says ten times:

اللّٰهُ لَا إِلٰهَ إِلَّا هُوَ وَحْدَهُ لَا شَرِيْكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ يُحْيِي وَيُمِيْتُ وَهُوَ عَلٰى كُلِّ شَيْءٍ قَدِيْرٌ

(None is worthy of worship but Allah, He is alone, He has no partner, His is the Kingdom, for Him is all the Praise, He alone gives life, and He alone gives death, and He has power over everything.)

then ten virtues are recorded for him, ten sins are erased, his rank is raised by ten degrees, and he is protected from every undesirable thing and from Shaitan (Satan) for that day. He will not be taken to account for any sin except polytheism.”

In another narration, the words:
بِيَدِهِ الْخَيْرُ (“In Whose Hand is all that is good”)
are in place of:
يُحْيِي وَيُمِيتُ (“He alone gives life and He alone gives death”).

It is also narrated that whoever says this phrase after the Asr prayer receives the reward as if he has worshipped for the whole night, and whoever says it after Fajr receives the reward for the entire day.

Reference: Tirmidhi, ’Amalul Yaumi wal Lailah by Nasai