Abu Dawud Book 2, Hadith Number 1213.
Narated By Abdullah ibn Umar : Abdullah ibn Dinar said: The sun set when I was with Abdullah ibn Umar. We proceeded, and when we saw that the evening came, we said prayer. He went on travelling until the twilight disappeared and the stars became thick. He then slighted and combined the two prayers. Then he said: I saw the Apostle of Allah (PBUH); when he hastened his travelling, he would pray like this prayer of mine. He said: He would combine the two prayers after the passing of a part of night. AbuDawud said: This has been transmitted by Asim ibn Muhammad from his brother on the authority of Salim and this has also been narrated by Ibn AbuNajih from Isma’il ibn AbdurRahman ibn Dhuwayb saying that Ibn Umar would combine the two prayers after the disappearance of twilight.