Sunan Abu Dawud
1213
Sahih+2

This tradition has also been transmitted by Ibrahim b. Musa al-Razi, from 'Isa, on the authority of Ibn Jabir to the same effect. Abu Dawud said:'Abd Allah b. al-'Ala' narrated on the authority of Nafi' saying: When the twilight was about to disappear, he alighted and combined both (the prayers)

SahihAl-Albani
SahihMuhammad Muhyi Al-Din Abdul Hamid