(a) Sayyiduna Ibn Abbas (may Allah be pleased with him) said that it is not permissible for anyone to send salah (blessings) upon anyone else except upon the Prophet (peace and blessings be upon him).
(b) Imam al-Bayhaqi (may Allah have mercy on him) said: By salah, Ibn Abbas (may Allah be pleased with him) meant the special salutation that is recited along with his name; this was because of the greatness of the Prophet (peace and blessings be upon him). As for salah upon anyone other than him, it is in the sense of supplication and blessing, and that is permissible for others besides the Prophet (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2876