Anas bin Maalik (radhi Allaahu anhu) said:
Verily the Prophet (salla Allahu alaihi wa sallam) said:
Whoever sends Salah upon me once Allah will send ten upon him and remove ten of his sins and raise his rank by ten levels
Imam Ahmad in his Musnad
Imam Bukharee in Adabul Mufrad #642 and #643.
Jami Sagheer #6359 authenticated by Al Abaanee