Sunan Ibn Majah 470
سنن ابن ماجه#470SahihIt was narrated that 'Umar bin Khattab said:"The Messenger of Allah said: 'There is no Muslim who performs ablution and does it well, then says: Ashhadu an la ilaha illallah, wa ashhadu anna Muhammadan `abduhu wa rasuluhu (I bear witness that none has the right to be worshipped but Allah, and I bear witness that Muhammad is His slave and Messenger),' (except that) eight gates of Paradise will be opened for him, and he will enter through whichever one he wants
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Fouad Abd al-BaqiSahih
Zubair Ali ZaiSahih Muslim
In-book reference: 1:204