Musannaf Ibn Abi Shayba 36:90

Abu Usama narrated to us, from Abu Hilal, from Qatada, from Sa'id ibn al-Musayyib, who said: I said to him, "What is the difference between the first emigrants (Muhajirun) and the later ones?" He said, "The difference between them is the two Qiblas. Whoever prayed with the Messenger of Allah (peace and blessings of Allah be upon him) facing the two Qiblas, he is among the first emigrants."