سورة المائدة

The Messenger of Allah (peace be upon him and his family): Abdullah ibn Salam asked the Prophet (peace be upon him and his family) and said: ... "O Muhammad! Inform me about a messenger who is neither from humans, nor from the jinn, nor from wild animals." He said: "فَبَعَثَ الله غُراباً یَبْحَثُ فِی الْأَرْضِ [Then Allah sent a crow digging up the earth]." He said: "You have spoken truly."

Imam Sajjad (peace be upon him): And the sins that bring about regret are killing a person whom Allah has forbidden [to kill], for He has said: وَ لا تَقْتُلُوا النَّفْسَ الَّتِی حَرَّمَ الله [And do not kill the soul which Allah has forbidden]; and Allah said in the story of Qabil: when he killed Habil and was unable to bury him, his rebellious soul made the killing of his brother appear good in his eyes, and in the end he regretted. And [among the other sins are] cutting off ties of kinship to the point that they turn away from him [as if such kin never existed], and abandoning the prayer until its time passes, and abandoning the will, and not returning what is owed to those who have been wronged, and withholding zakat until death arrives and the tongue is prevented from speaking.