Musnad Abu Ya'la al-Mawsili 115:146

Abu Musa narrated to us, Ibn Abi ‘Adi and ‘Abd al-A’la narrated to us, from Sa’id, from Qatadah, from Anas: That the Prophet (peace and blessings of Allah be upon him) would not raise his hands in any supplication except in the prayer for rain (Istisqa’), until the whiteness of his armpits could be seen. However, ‘Abd al-A’la said: "Until the whiteness of his armpits or the whiteness of his armpit could be seen."