Sunan Abi Dawud 1213
سنن أبي داود#1213SahihThis 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)
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Muhyi Al-Din Abdul HamidSahih
Zubair Ali ZaiIsnaad Sahih
In-book reference: 4:16