Sunan an-Nasa'i 148
سنن النسائي#148SahihIt was narrated that 'Umar bin Al-Khattab said:"The Messenger of Allah (ﷺ) said: 'Whoever performs Wudu' and does it well, then says: "Ashhadu an la ilaha ill-Allah was ashhadu anna Muhammadan 'abduhu wa rasuluh (I bear witness that there is none worthy of worship except Allah, and I bear witness that Muhammad is his slave and Messenger)," eight gates of Paradise will be opened for him, and he may enter through whichever one he wishes
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih Muslim
In-book reference: 1:149