Reference : Hisn al-Muslim 255

وَقَالَ صلى الله عليه وسلم: مَنْ قَالَ (لاَ إِلَهَ إِلاَّ اللَّهُ وَحْدَهُ لاَ شَرِيكَ لَهُ، لَهُ الْمُلْكُ، وَلَهُ الْحَمْدُ، وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ) عَشْرَ مِرَارٍ، كَانَ كَمَنْ أَعْتَقَ أَرْبَعَةَ أَنْفُسٍ مِنْ وَلَدِ إِسْمَاعِيلَ
Allah's Messenger (ﷺ) said: Whoever says:
Lā ilāha illallāh
waḥdahu lā sharīka lah, 
lahu ‘l-mulku wa lahu ‘l-ḥamd 
wa huwa `alā kulli shay'in Qadīr.
ten times, will have the reward for freeing four slaves from the Children of Isma'il.

Allah's Messenger (ﷺ) said: Whoever says:

None has the right to be worshipped but Allah alone, Who has no partner. 
His is the dominion and His is the praise, and He is Able to do all things.

ten times, will have the reward for freeing four slaves from the Children of Isma'il.

Reference:
Al-Bukhari 7/67, Muslim 4/2071, see also invocation no. 93 of this book.

Chapter: Excellence of Tasbih, Tahmid, Tahlil, and Takbir