Sahih Muslim 649f

The Prophet (may Allah's peace and blessings be upon him) informs that the reward of the prayer that a man offers in congregation exceeds the reward of the prayer he offers in his home or market, i.e. the prayer which he offers alone, as inferred by comparing it to the congregational prayer, and because the prayers offered at home or in the market are performed individually in most cases.

Congregational prayer exceeds the prayer one offers alone by the amount of twenty five times. His statement: "That is because," if "that" here refers to the virtue of prayer in congregation over praying alone, then this necessitates that this virtue is restricted to offering the prayer in congregation in the mosque. His statement: "Because if he," i.e. the person, if he performs ablution thoroughly, i.e. properly by doing the Sunnah acts and observes its etiquettes, then sets out in the direction of the mosque, with nothing causing him to go out but the prayer (if something else causes him to go out, or he goes out because of something else along with the prayer, then he will miss the following merits: He will not take a step except that he is raised one degree by it, and one sin is removed from his account, i.e. the minor sins involving rights of Allah, the Exalted.

When he finishes his prayer, the angels continue to supplicate Allah for him, seeking mercy and forgiveness for him, as long as he is in his place of prayer, i.e. sitting therein. It is also probable that it means: as long as he remains in it (the place) even lying down, and does not invalidate his ablution. The praying person remains in prayer as long as he is waiting for the prayer," i.e. the period of time during which he is waiting for it.