Musannaf Ibn Abi Shayba 3:1787

Abu Bakr narrated to us, he said: Isma'il ibn 'Ulayyah narrated to us, from Kathir ibn Shinzhir, from 'Ata', from Abu Hurayrah, who said: "If a man reaches a group of people while they are sitting at the end of their prayer, he has entered into the [reward of] multiplication. And if he reaches them when the Imam has already performed the taslim, but they have not yet dispersed, he has entered into the [reward of] multiplication." And 'Ata' said: "It used to be said: If he leaves his house intending them [to join them], whether he catches them or does not catch them, he has entered into the [reward of] multiplication."