It was the practice of the Prophet (peace and blessings be upon him):
"When he (peace and blessings be upon him) asked Allah for something, he would turn the inner side of both his palms towards Him."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4737
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[حم] عن السائب بن خلاد١. الضعيفة ٤١٩٩.