An-Nu‘man b. Bashir reported God’s messenger as saying, “Supplication is worship.” He then recited, “And your Lord said, If you call on me I will aswer you.”*
*Qur’an, xl, 60.
Ahmad, Tirmidhi, Abu Dawud, Nasa'i and Ibn Majah transmitted it.
Jabir reported God’s messenger as saying, “Do not invoke curses on yourselves, or on your children, or on your possessions lest you happen to do it at a time when God is asked for something and grants your requests.”
Abu Darda’ reported God’s messenger as saying, “A Muslim’s supplication for an absent brother receives an answer. An angel is stationed at his head, and as often as he makes supplication that his brother may be given what is good the angel who is put in charge of him says, ‘Amen, and may you receive the like’.”