Musnad Ahmad 30:1979

Ibn Mahdi narrated to us, from Malik, from al-Mughirah ibn Abi Burdah, from Abu al-Zinad, from al-A’raj, from Abu Hurayrah, that the Messenger of Allah (ﷺ) said: "The prayer of one of you in congregation is twenty-five degrees better than his prayer in his house or in his marketplace. That is because if one of you performs ablution and perfects it, then goes to the mosque, not intending anything except the prayer, he does not take a step except that he is raised by it a degree and a sin is removed from him by it, until he enters the mosque. When he enters the mosque, he is in prayer as long as the prayer detains him. And the angels pray for one of you as long as he is in the place where he prayed, saying: 'O Allah, forgive him, O Allah, have mercy on him,' as long as he does not commit a hadath (break his ablution) in it."