Al-Matalib al-'Aliyah 740 — Shortening the prayer while travelling

It is narrated from Abu Ma'bad (may Allah have mercy on him) that he said: We were with Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) on a journey. When the sun set, it was said to him: "O Abu al-Abbas! The prayer (its time has come)?" He said: "We are a travelling people (and we have the concession to combine prayers on a journey)." Then he kept travelling until he reached Marr al-Zahran and halted there; and between the place where he had been told about the prayer at sunset and where he finally halted, there was a distance of two farsakhs (about eleven kilometres).
The chain of this hadith is sahih (authentic), and Imam al-Nasa'i recorded it in al-Sunan al-Kubra (hadith: 604).