Imam al-Shafi'i (may Allah have mercy on him) said: I consider it disliked to call Muharram "Safar"; it should be called Muharram. And I consider it disliked to call Muharram "Safar" only because the people of the Jahiliyyah used to count the two months, Muharram and Safar, both as Safar, and they used to move Muharram and Safar forward and back; one year they would perform Hajj in the month of Hajj and another year in some other month, and they would say: If we have mistaken the place of the sacred month this year, we will get it right next year. Allah Almighty says: "The postponing (of a sacred month) is only an increase in disbelief" [Surah al-Tawbah: 37]. And the Messenger of Allah (peace and blessings be upon him) said: "Time has come round to its form on the day Allah created the heavens and the earth"; so no month is to be moved forward or back. And the Prophet (peace and blessings be upon him) named it Muharram.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9772