It was the practice of the Prophet (peace and blessings be upon him):
"When he (peace and blessings be upon him) prostrated, he would hold his arms so far away from his sides that the whiteness of his (peace and blessings be upon him) armpits could be seen."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4738
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[د] عن جابر. حم ٣/٢٩٥، م ١/٥٤ - ميمونة.