Majma al-Zawaid — Hadith 2972

Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrated: When the Prophet (peace and blessings be upon him) was on a journey and had to travel quickly, and he set off before the sun had declined, he would delay the Zuhr prayer until the beginning of the time of Asr arrived; then he would dismount and offer both prayers together. Then he would delay the Maghrib prayer until the twilight had disappeared; then he would dismount, and he (peace and blessings be upon him) would offer both the Maghrib and Isha prayers together.
Imam al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains a narrator, Abu Ma'shar Najih, about whom there has been much criticism, though some have declared him thiqah (trustworthy).