مُوَطَّأُ مَالِكٍ
Muwatta Malik 158
Prayer
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ لاَ يَزِيدُ عَلَى الإِقَامَةِ فِي السَّفَرِ إِلاَّ فِي الصُّبْحِ فَإِنَّهُ كَانَ يُنَادِي فِيهَا وَيُقِيمُ وَكَانَ يَقُولُ إِنَّمَا الأَذَانُ لِلإِمَامِ الَّذِي يَجْتَمِعُ النَّاسُ إِلَيْهِ .
Yahya related to me from Malik from Nafi that on a journey Abdullah ibn Umar did no more than the iqama, except for subh, when he called both the adhan and the iqama. Abdullah ibn Umar used to say, "The adhan is for an imam whom people join ."
Arabic/English book reference: Book 3, Hadith 12