Sahih Muslim Book 6, Hadith 82
When any one of you enters the mosque, he should say:" O Allah! open for me the doors of Thy mercy" ; and when he steps out he should say: 'O Allah! I beg of Thee Thy Grace." (Imam Muslim said: I heard Yahya saying: I transcribed this hadith from the compilation of Sulaiman b. Bilal.)
A person entered the mosque, while the Messenger of Allah (ﷺ) was leading the dawn prayer. He observed two rak'ahs in a corner of the mosque, and then joined the Messenger of Allah (ﷺ) in prayer. When the Messenger of Allah (ﷺ) had pronounced salutations (he had concluded the prayer), he said: O, so and so, which one out of these two prayers did you count (as your Fard prayer), the one that you observed alone or the prayer that you observed with us?
A hadith like this has been narrated from the Messenger of Allah (ﷺ) by Abu Usaid.