Lady Maymunah, the wife of the Noble Prophet (peace be upon him), said: When the Messenger of Allah (peace be upon him) performed prostration, he would keep both his hands so open (separated from his sides) that the whiteness of his armpits could be seen from behind, and when he sat, he would lean on his left thigh.
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1369
(Commentary on Hadiths 1366 to 1369) From all these hadiths, it is established that in the state of prostration (sajdah), the hands and arms should be kept away from the sides. Therefore, in prostration, one should not keep the hands stuck to the ribs.