From Sahih Muslim (Hadith 2171):
A hadith like this has been transmitted by Hafsa on the authority of Umm 'Atiyya with the exception (of these words that the Prophet asked them to wash her dead body):" three times, five times, seven times, or more than that, if you deem fit:" Hafsa (further) said on the authority of Umm 'Atiyya: We braided (the hair) of her head in three plaits
Reference: Muslim 2171