Sahih Bukhari Volume 003, Book 027, Hadith Number 032.
Narated 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)."