All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.
It is a Sunnah that a Muslim prays for the household of the Prophet as he himself used to say after reciting the Tashahhud while seated in his prayer: "Allaahumma salli ‘ala Muhammad wa‘ala 'ahlibaytihi wa'azwaajih… (May Allaah confer peace upon Muhammad, his household and his wives)." [Ahmad and Abdur-Razzaaq in his Musannaf]
An-Nawawi who was a Shaafi‘i scholar, said that the Shaafi‘i scholars are of the opinion that praying for a particular person in this manner is not permissible. This prayer is only for the prophets. This is so because the righteous predecessors made this privilege exclusive to the prophets, may Allaah exalt their mention, just as the phrase "‘Azza wa Jall" (may He be exalted and glorified) is specific to Allaah The Almighty. Therefore, just as we cannot say "Muhammad, may he be exalted ", even if he is exalted, we cannot say, "Abu Bakr ” , even though the meaning is correct.
Allaah Knows best.