It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that he (peace and blessings be upon him) said: "When the iqamah has been called, there is no prayer except the obligatory one." On one occasion he (peace and blessings be upon him) said: "When the prayer has begun."
(b) Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said: "When the iqamah for the prayer has been called, there is no prayer except the obligatory one."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4546