Sayyidah Umm Salamah (may Allah be pleased with her) reports this saying of the Prophet (peace and blessings be upon him): "After every two rak'ahs, the tashahhud is to be recited, and peace is to be sent upon the messengers and also upon the righteous servants of Allah who follow them."
Imam al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains a narrator, Ali bin Zayd, about whom there is disagreement over whether he may be used as evidence, and he has been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2839