“Whoever consistently observes twelve units of Sunnah prayer, Allah will build a house for him in Paradise: four before ẓuhr, two after it, two after maghrib, two after ʿishāʾ, and two before fajr.”

Reported by at-Tirmidhī (no. 414), an-Nasāʾī (no. 1794), and Ibn Mājah (no. 1140), from the narration of ʿĀʾishah (may Allah be pleased with her).
Authenticated in Ṣaḥīḥ al-Jāmiʿ (no. 6183), and Ṣaḥīḥ at-Targhīb wa at-Tarhīb (no. 580).

Brief Explanation of the Hadith

Prayer is the pillar of religion and the greatest of physical acts of worship. A Muslim should give it due care and strive for its perfection and completeness. If any deficiency occurs in it, then voluntary prayers have been legislated to make up for it and to raise the ranks of the one who performs them. Voluntary prayers are among the acts of devotion loved by Allah, the Exalted. Among these voluntary prayers are the regular Sunnah prayers (as-sunan ar-rawātib), which the Prophet (may Allah’s peace and blessings be upon him) strongly encouraged by saying: “Whoever perseveres,” meaning: remains constant, committed, and diligent, striving against his own self to perform them daily — Allah will build for him, because of these rak‘ahs, a house in Paradise — that is, a magnificent palace filled with various forms of bliss. These are twelve rak‘ahs performed daily and nightly from the non-obligatory prayers, apart from the obligatory ones: four rak‘ahs before ẓuhr, two after it, two after maghrib, two after ‘ishā’, and two before fajr. These are confirmed Sunnahs, observed at different times — some before the obligatory prayers and some after them.
Some scholars have offered a beautiful reasoning for why some of these supererogatory prayers (nawāfil) are performed before the obligatory prayers and others after. They said: pre-obligatory (qabliyyah) prayers prepare and energize the soul to perform the obligatory prayer with focus and completeness. As for the post-obligatory (ba‘diyyah) prayers, they serve to compensate for any shortcomings that may have occurred during the obligatory prayer.
This tremendous reward mentioned in the ḥadīth is for the one who consistently maintains these prayers and regularly performs them — not for the one who observes them one day and neglects them another.