It is narrated from Nafi' that Sayyiduna Ibn Umar (may Allah be pleased with them both) would say the takbir three times in the adhan and the testimony three times, and sometimes, when he said 'Hayya 'ala al-falah' (Come to success), he would say after it: 'Hayya 'ala khayr al-'amal' (Come to the best of deeds).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1991