Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "When the iqamah is called, let none of you come running to the prayer; rather let him come walking with calm and tranquillity. Let him pray what he catches and make up what has passed."
(b) Abu Rafi' narrates the like of this from Sayyiduna Abu Hurayrah (may Allah be pleased with him). Those who narrate it as "fa-atimmu" (complete it) are more numerous, better at memorising and closer in companionship to Sayyiduna Abu Hurayrah (may Allah be pleased with him), so their version is preferable. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3629