It is narrated from Sayyiduna al-Mughirah bin Shu'bah (may Allah be pleased with him) that when the Prophet (peace and blessings be upon him) finished the prayer, he would say: "There is no true god except Allah" - then the narrator mentioned the complete hadith.
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 18031