Narrated by Abu Huraira (Radiyallahu `anhu) that the Prophet (Sallallaahu `alayhi wa sallam) said,
“If any one of you says, ‘Ameen’ and the angels in the heavens say ‘Ameen’ and the former coincides with the latter, all his past sins will be forgiven.”

Narrated Anas bin Malik (Radiyallaahu `anhu),
“Once in the lifetime of the Prophet (p.b.u.h) the people were afflicted with drought (famine). While the Prophet was delivering the Khutba on a Friday, a Bedouin stood up and said, “O, Allah’s Apostle! Our possessions are being destroyed and the children are hungry; Please invoke Allah (for rain)”.

Narrated by Sahel Ibn Sa’ad (radiallaahu anhu), that the Messenger of Allah (sallallaahu alaihi wasallam) said,
“Two will not be rejected, Supplication when the Adhan (call of prayer) is being called, and at the time of the rain”

On the authority of Umm Darda (Radiyallahu `anhuma) it is reported that Prophet (Sallallaahu `alayhi wa sallam) said:
“…the dua of a muslim for his brother (in Islam) in his absence is readily accepted, and an angel is appointed to his side, whenever he makes a beneficial dua for his brother the appointed angel says “Ameen and may you also be blessed with the same.”

The Prophet (Sallallaahu `alayhi wa sallam) said,
“No one offers a supplication without Allah bringing him what he asks or keeping away from him a like amount of evil, provided he does not ask for something sinful or for breaking ties of relationship.”