Al-Aswad narrates: I asked Abu Mahdhurah (may Allah be pleased with him): "How did you give the adhan for the Prophet (peace and blessings be upon him)? What did you say at the end of your adhan?" He said: "I used to repeat the adhan (in pairs), and I also repeated the iqamah, and I would say 'La ilaha illallah' at the end of my adhan."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6710