Musnad Ahmad 1060:98

Yunus ibn Muhammad and Hujayn narrated to us, they said: Layth ibn Sa’d narrated to us, from Abu al-Zubayr, from Sufyan ibn ‘Abd al-Rahman, from ‘Asim ibn Sufyan al-Thaqafi, that they went out on the expedition of al-Salasil. The expedition passed them by, so they remained stationed (in ribat), then they returned to Mu’awiyah, and with him were Abu Ayyub and ‘Uqbah ibn ‘Amir. ‘Asim said: "O Abu Ayyub, the expedition passed us by this year, and we have been informed that whoever prays in the mosque"—and Hujayn said: "the four mosques"—"his sins will be forgiven." He (Abu Ayyub) said: "O son of my brother, shall I guide you to something easier than that? I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Whoever performs ablution as he was commanded, and prays as he was commanded, his previous deeds will be forgiven.'" He said: "Is that so, O ‘Uqbah?" He said: "Yes."