Sahih Muslim 1695
صحيح مسلم#1695SahihNu'man b. Salim reported with the same chain of transmitters:He who observed twelve voluntary rak'ahs, a house will be built for him in Paradise
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 6:125