The Merit of Salawat

Imam Sadiq [a.s] said: ‘Every invocation sought from Allah, Almighty and Glorious is barred by the sky unless it is coupled with 'Salawat', benediction upon the Prophet and his progeny.’ (The expression of 'Salawat' means: O God bless Muhammad and his descendants).

Usul-i-Kafi, vol. 2, p. 493

Source: al-shia.org