Sahih Bukhari | Volume 3 | Hadith Number 32

Narrated by Zaid bin Aslam
From his father, I was with Ibn 'Umar on the way to Mecca, and he got the news that Safiya bint Abu Ubaid was seriously ill. So, he hastened his pace, and when the twilight disappeared, he dismounted and offered the Maghrib and 'Isha prayers together. Then he said, "I saw that whenever the Prophet had to hasten when traveling, he would delay the Maghrib prayer and join them together (i.e. offer the Maghrib and the 'Isha prayers together)."