Mishkat al-Masabih 3181
Ibn ‘Abbas said:
Seven classes of women are prohibited by reason of consanguinity and seven by reason of relationship by marriage. He then recited, "Prohibited to you are your mothers ...” (Al-Qur’an 4:23).

Bukhari transmitted it.
عَنِ ابْنِ عَبَّاسٍ قَالَ: حُرِّمَ مِنَ النَّسَبِ سَبْعٌ وَمِنَ الصِّهْرِ سَبْعٌ ثُمَّ قَرَأَ: (حُرِّمَتْ عَلَيْكُم أُمَّهَاتكُم)
الْآيَة. رَوَاهُ البُخَارِيّ