Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.039C.

This involves only obligatory, voluntary, or funeral prayers, not the supplications (du’a). This is based on Allah’s statement, “O you who believe. When you get up to perform a prayer (salah) wash your face and your arms up to the elbows and wipe your head and feet to the ankles.” Also, the Messenger of Allah, upon whom be peace, said, “Allah does not accept a prayer (that was performed while the person) was not in a state of purity. Nor does he accept charity from misappropriated booty.” (Related by “the group,” except for al-Bukhari.)