Hadith 4:240
Muhammad b. an-Nu‘man who traced the tradition back to the Prophet reported him as saying, “If anyone visits the grave of his parents, or of one of them, every Friday he will be granted forgiveness and it will be recorded as filial piety.” Baihaqi transmitted it in mursal form in Shu'ab al-iman .
وَعَنْ مُحَمَّدِ بْنِ النُّعْمَانِ يُرْفَعُ الْحَدِيثَ إِلَى النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «مَنْ زَارَ قَبْرَ أَبَوَيْهِ أَوْ أَحَدِهِمَا فِي كُلِّ جُمُعَةٍ غُفِرَ لَهُ وَكُتِبَ بَرًّا» . رَوَاهُ الْبَيْهَقِيُّ فِي شعب الْإِيمَان مُرْسلا
Original Reference: Mishkat al-Masabih 1768