Abu Dawud Book 2, Hadith Number 1205.

Chapter : On the combination of two prayers at a time.

Narated By Ibn ‘Umar : The Apostle of Allah (PBUH) never combined the sunset and the night prayers while on journey except once.

Abu Dawud said: This has been narrated by Ayyub from Nafi’ from Ibn ‘Umar as a statement of Ibn ‘Umar. Ibn ‘Umar was never seen combining these two prayers except on the night he was informed of the death of Safiyyah. This tradition narrated by Makhul from Nafi’ indicates that he (Nafi’) saw Ibn ‘Umar doing so once or twice.