Sahih Al Bukhari Hadith Number 807

Narrated `Abdullah bin Malik bin Buhaina: Whenever the Prophet (peace be upon him) used to offer prayer he used to keep arms away (from the body) so that the whiteness of his armpits was visible.

10  Call to Prayers (Adhaan)
Hadith number 807
Volume no 1
Chapter: 130 (During the prostrations one should keep one’s arms away from one’s sides and the abdomen should be kept away from the thighs.)