Out of that affection and thoughtfulness he felt towards the faithful, the Prophet (PBUH) asked Allah to forgive their mistakes. Allah's commands to the Prophet (PBUH) on that subject take the following form:
In another verse, Allah told the Prophet (PBUH) to pray for the faithful:
... Pray for them. Your prayers bring relief to them. Allah is All-Hearing, All-Knowing. (Surat at-Tawba: 103)
As this verse reveals, the Prophet (PBUH)'s prayers brought the faithful peace and tranquility. It must not be forgotten that only Allah gives peace and tranquility to the heart. Allah makes the prayers of His Messenger (PBUH), whom He has appointed as guardian and protector of the faithful, a means whereby they come by comfort and ease. Our Lord's love and compassion, and His protection and mercy for the faithful is most clearly represented in the morality of the Prophet (PBUH).
Those who do good will have the best and more! Neither dust nor debasement will darken their faces. They are the Companions of the Garden, remaining in it timelessly, for ever.(Surah Yunus: 26)
source : tebyan