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