عَنْ أَبِي عَبْدِ اللٌّهِ (ع): قَالَ جَاءَ رَجُلٌ إِلَى النَّبِيِّ (ص) فَقَالَ: يَا رَسُولَ اللٌّهِ مَنْ أَبَرُّ؟ قَالَ أُمَّكَ. قَالَ ثُمَّ مَنْ؟ قَالَ أُمَّكَ. قَالَ ثُمَّ مَنْ؟ قَالَ أُمَّكَ. قَالَ ثُمَّ مَنْ؟ قَالَ أَبَاكَ.
Imam as-Sadiq (peace be upon him) relates that once a person approached the Noble Prophet (peace be upon him and his family) and asked: “O’ Prophet of Allah! Towards whom should I exhibit goodness and kindness?” The Noble Prophet replied: “Towards your mother.” The man then asked: “And after that towards whom?” The Noble Prophet again said: “Your Mother.” He asked again: “And then?” Once again, he replied: “Your mother.” For the fourth time the man asked: “And then?” This time he said: “(Then towards) Your father.”
Biharul Anwar, Volume 74, Page 49