It is narrated from Nafi' that Sayyiduna Ibn Umar (may Allah be pleased with them both) would not give the adhan on his journeys, and he would say: 'Hayya 'ala al-falah' (Come to success), and sometimes he would say: 'Hayya 'ala khayr al-'amal' (Come to the best of deeds).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1992