'Ata' ibn Yasar said that a man came to Ibn 'Abbas and said, "I asked a woman to marry me and she refused to marry me. Another man asked her and she agreed to marry him. I became jealous and killed her. Is there any way for me to repent?" He asked, "Is your mother alive?" "No," he replied. He said, "repent to Allah Almighty and try to draw near Him as much as you can."
So I went and asked Ibn `Abbas why he inquired about the man's mother. He replied:
"I don't know of a deed closer to Allah, Exalted and Majestic, other than dutifulness to the mother."

حَدَّثَنَا سَعِيدُ بْنُ أَبِي مَرْيَمَ، قَالَ: أَخْبَرَنَا مُحَمَّدُ بْنُ جَعْفَرِ بْنِ أَبِي كَثِيرٍ قَالَ: أَخْبَرَنِي زَيْدُ بْنُ أَسْلَمَ، عَنْ عَطَاءِ بْنِ يَسَارٍ، عَنِ ابْنِ عَبَّاسٍ، أَنَّهُ أَتَاهُ رَجُلٌ فَقَالَ: إِنِّي خَطَبْتُ امْرَأَةً، فَأَبَتْ أَنْ تَنْكِحَنِي، وَخَطَبَهَا غَيْرِي، فَأَحَبَّتْ أَنْ تَنْكِحَهُ، فَغِرْتُ عَلَيْهَا فَقَتَلْتُهَا، فَهَلْ لِي مِنْ تَوْبَةٍ؟ قَالَ: أُمُّكَ حَيَّةٌ؟ قَالَ: لاَ، قَالَ: تُبْ إِلَى اللهِ عَزَّ وَجَلَّ، وَتَقَرَّبْ إِلَيْهِ مَا اسْتَطَعْتَ. فَذَهَبْتُ فَسَأَلْتُ ابْنَ عَبَّاسٍ: لِمَ سَأَلْتَهُ عَنْ حَيَاةِ أُمِّهِ؟ فَقَالَ: إِنِّي لاَ أَعْلَمُ عَمَلاً أَقْرَبَ إِلَى اللهِ عَزَّ وَجَلَّ مِنْ بِرِّ الْوَالِدَةِ.